We report the first successful preparation of thin films of Y-Ba-Cu-O superconductors using pulsed excimer laser evaporation of a single bulk material target in vacuum. Rutherford backscattering spectrometry showed the composition of these films to be close to that of the bulk material. Growth rates were typically 0.1 nm per laser shot. After an annealing treatment in oxygen the films exhibited superconductivity with an onset at 95 K and zero resistance at 85 and 75 K on SrTiO3 and Al2O3 substrates, respectively. This new deposition method is relatively simple, very versatile, and does not require the use of ultrahigh vacuum techniques.

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.

We present results of temperature and magnetic field dependent resistivity ρ(H,T) and bulk magnetization M(H,T) measurements on post-annealed La0.7Ca0.3MnO3 thin films that were grown via pulsed-laser deposition. Both the resistivity and the anomalously large negative magnetoresistance peak near the ferromagnetic ordering temperature (Tc=250 K), with Δρ/ρ0=−85% at 50 kOe. A clear correlation is found between ρ and M that is described by the phenomenological expression ρ(H,T)∝exp[−M(H,T)/M0]. This correlation reflects the important interplay between transport and magnetism in this system, and suggests that the transport below Tc involves polaron hopping.

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Using Rutherford backscattering technique, we have measured the angular distribution of the composition and thickness of the Y-Ba-Cu oxide film deposited by firing excimer laser (30 ns, 248 nm) pulses at a stoichiometric Y1Ba2Cu3O7−x pellet. The angular distribution consisted of two distinct components: one a cos θ component, a result of evaporation, and the other a highly forward directed component, a result of a secondary ejection process. The evaporated component is nonstoichiometric, as one would expect, whereas the forward-directed component has a composition close to that of the pellet. Further, the forward-directed stoichiometric component increases with the laser energy density in comparison with the evaporated component. These observations are discussed in the context of current models of laser-induced material ejection at surfaces.The laser energy dependence of the deposition is of critical importance in controlling the film stoichiometry.

The frequency-dependent conductivity of laser-deposited YBa2Cu3C>7-s thin films shows an onset of midinfrared absorption at -140 cm -1 and structure in the 400-500-cm ~! region. These low-energy absorptions occur both above and below 7V, making them unlikely to be the superconducting gap in the usual BCS sense. The absorption across the gap is weak because the high-r c materials are in the clean limit; this weak absorption is masked by the midinfrared absorption.