Thin-film stabilization of metastable phases in the Sm1-xSrxCuOy system

The pulsed laser deposition (PLD) technique has been utilized for thin film synthesis of new metastable phases in the Sm[sub 1-x]Sr[sub x]CuO[sub y] (y = 2.5-0.5x + [delta]) system. The films are grown epitaxially on (100)-oriented SrTiO[sub 3] substrates in a low-pressure oxygen ambient with in situ monitoring using reflection high-energy electron diffraction (RHEED). X-ray diffraction and transmission electron microscopy are used for structural characterization of the films. Although the synthesis of SmCuO[sub 3-[delta]]-the structural analogue of the defect perovskites LaCuO[sub 3-[delta]] (0 [le] [delta] [le] 0.5) - have not been successful, perovskite-type phases have been stabilized with Sr substitution over a relatively wide composition range (0.4 [le] x [le] 0.66). With further increase in the Sr content, an ordered 2[radical]2a[sub p] x 2[radical]2a[sub p] x a[sub p] structure with mixed copper valence has been synthesized for x [approximately] 0.75. Finally, for terminal Sr concentrations (x [ge] 0.9), the doped infinite-layer-type SrCuO[sub 2] phases are obtained. The transport properties of the Sm[sub 1-x]Sr[sub x]CuO[sub y] films are sensitive to the oxygen concentration used during deposition and subsequent cooldown. For films cooled in 760 Torr of O[sub 2], a systematic decrease in the resistivity is observed as a function of Srmore » content, with the lowest resistivity and metallic temperature dependence being observed for the ordered structure. However, films with higher Sr concentrations, with the infinite-layer structure, exhibit relatively high resistivity under these annealing conditions. A significant decrease in the resistivity, along with semimetallic temperature dependence, is observed for these (Sm-doped) films when they are vacuum annealed prior to cooldown. No evidence of superconductivity has been detected in any of the Sm[sub 1-x]Sr[sub x]CuO[sub y] films down to 5 K. 19 refs., 6 figs.« less