Investigation for anormalous superconducting critical temperature behavior of superconductor/normalconductor alternately-layered thin films

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We fabricated M_gB2 thin films with various thickness on several kinds of substrates. We found that the superconducting critical temperature(T_c) of the thicker films have higher Tc than those of the thinner ones. The reason of these lower T_c for the thinner films was their poorer crystallinity. These results suggest that the Tc of the superconductor/normalconductor alternately-layered thin film was influenced by the crystallinity of the superconducting layer. However, since the columnar M_gB2 crystals of the M_gB2 layers separated by the inserted Ni-layers were confirmed to grow continuously and epitaxially through the separating Ni-layers from a cross-sectional transmission electron microscope observation, other factors are also to govern the Tc of the M_gB2/Ni alternately-layered thin film. We also found that the M_gB2 thin films have a very high critical current density(J_c) over 10 MA/_<cm> 2 at 10 K when they were grown with small grain size of few tens nm. We believe that such a high Jc value may open a new way to fabricate a practical superconducting wire.