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

我们在几种衬底上制备了不同厚度的M_gB_2薄膜。发现厚膜的超导临界温度比薄膜的高。薄膜的结晶性较差是薄膜的T_c较低的原因。这些结果表明，超导体/正常导体交替层状薄膜的Tc的超导层的结晶度的影响。然而，由于从横截面透射电子显微镜观察证实了被插入的Ni层分离的M_gB2层的柱状M_gB2晶体通过分离的Ni层连续地和外延地生长，所以其它因素也决定了M_gB2/Ni交替层叠薄膜的Tc。在10 K时，当晶粒尺寸为几十nm时，M_gB_2薄膜具有很高的临界电流密度（J_c），超过10 MA/<cm>_2。我们相信如此高的Jc值可能会为制造实用的超导导线开辟一条新的途径。

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.