Superconducting and Semiconducting Thin films of La 123 and 123 and their Superlattices
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Superconducting and Semiconducting Thin films of La123 and Y123 and their Superlattices L. Ganapathi, Ashok Kumar and J. Narayan Department of Materials Science and Engineering, North Carolina State University Raleigh, NC 27695-7916 0. W. Holland Solid State Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831
Abstract Thin films of Lal 2 3 and their superlattice with Y123 are deposited on various substrates for the first time. Thin films of La123 can be made semiconducting or superconducting within a small range of oxygen stoichiometry. A multiple target holder has been used for insitu processing of superlattice thin films. The ability of the La 12 3 compound to exhibit either semiconductivity or superconductivity has been utilized to construct semiconductor / superconductor superlattice structures with Y123 - X-ray diffraction studies have shown clearly the presence of two distinct type of layers in the superlattice films with a individual layer thickness of - 120 and 300 A, although this did not happen for - 24 A thickness of the individual layers. Rutherford backscattering experiments showed good quality of the films with channeling yields < 10%. Resistivity measurements showed a Tc (onset) = 90 K and Tc(zero) = 85 K for the Lal2 3 as well as superlattice thin films. INTRODUCTION LaBa 2 Cu 3 O7 _8 has been the most intriguing compound in the 123 family of superconductors 1-3. Its optimum properties are rarely obtained and the preparative conditions are quite complex to obtain a superconducting sample. Although initial reports identified it as a tetragonal superconductor, detailed X-ray and neutron diffraction studies 4,5 showed the existence of an orthorhombic phase and it has been believed that the orthorhombic phase is responsible for the high temperature superconductivity. We have recently shown that orthorhombic structure is not a sufficient criterion but it should be accompanied with proper hole concentration for achieving the superconductivity in this compound 6. Owing, probably, to the stringent preparative conditions for obtaining superconducting La123, extensive investigations in parallel with the Y123 compound are not undertaken despite the lower cost of La 2 03. Recently, several groups have made claims of reproducible production of superconducting Lal 2 3 compound 6-8. All these investigations highlight the ordering of the La and Ba similar to the Y 12 3 as essential for the orthorhombic structure and hence superconductivity of the compound. We have demonstrated that hole concentration is an essential parameter which must be optimized to observe superconductivity in the La 12 3 compound. These two compounds emphasize the importance of the hole concentration for the superconductivity of these oxides. Extremely fine control of the growth process has enabled the formation of various metastable phases through thin film technology. Layer by layer deposition of high-Tc oxide thin films have been successfully demonstrated following the discovery of high-Tc superconductivity 9-11. Recently,
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