Development of High Performances and Novel Functions in Low-Growth Temperature (Cu, C) Superconducting Film by Controlling Multi-layer Structure

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Thin film process and enhancement of properties of (Cu, C)-system [(Cu,C)Ba_2Ca_<n-1>Cu_nO_x:(Cu,C)-12(n-1)n] high temperature superconductors have been studied. Contrary to the necessity of high temperature and high pressure for fabrication of bulk ceramics of this system, thin films of (Cu,C)-1201 phase have successfully synthesized by utilizing pulsed laser deposition on SrTiO_3 substrate under CO_2 mixed atmosphere. The (Cu,C)-1201 films show technological merits of the low growth temperature of 500～600 ℃ and superconducting critical temperature about 50 K which is sufficient to applications of high-speed digital circuits. Though surface roughness of the 1201 films directly on the substrate exceeded c-axis lattice parameter of this phase, it has been improved up to sub-nano meter level by the insertion of SrCuO_2 buffer with infinite layer structure between the 1201 film and the substrate. The (Cu,C)-1201/SrCuO_2 structures exhibit similar superconducting properties. These results mean that it is satisfied the necessary conditions for the investigation of the effect of multi-stacking of 1201 and related layers on superconducting properties of it. In the present study, remarkable enhancements of critical temperature are achieved in [CaCuO_2/(Cu,C)-1201]×N (N : repetition number of fundamental structure) multi-layered structure. T_<c-onset> and T_<c(ρ=0)> reach～100 K and >70 K, respectively. It is also noteworthy that the growth temperature of such high-T_c specimens still stays around 500℃. Structural analysis of the high-T_c specimens has revealed the strain at their hetero-interfaces is the main origin of the enhancement of superconductivity.The achievements mentioned above shows that the novel superconducting film withtechnological merits : high T_c and low growth temperature, without toxic element has developed by adopting the hetero-interface effect to (Cu,C)-1201 layer, which also means the objective of this research is successfully achieved.