Evidence of disordered local structure of ZnO buffered MgB2 films via polarized extended X-ray absorption fine structure analysis

Herein, we investigated the mechanism by which the critical temperature (Tc) of a magnesium diboride (MgB2) superconductor is altered by the buffer layer. Zinc oxide (ZnO) layers with different thicknesses (28, 80, and 134 nm) were deposited epitaxially on a Hastelloy substrate beneath a similar to 1 mu m MgB2 film. To measure the local structural distortion of the boron plane, three different angle polarizations (0 degrees, 45 degrees, and 90 degrees parallel to the abaxis) on the Mg K-edge, extended X-ray absorption fine structure (EXAFS) measurements were performed. The EXAFS analyses revealed that the ZnO buffer layer caused the boron layer to disorient, leading to buckling between the Mg and B layers, which affected the charge transfer capability in the intra-layer structure. Furthermore, the uneven distribution of B atoms amid the surrounding Mg atoms resulted in an asymmetric crystal structure, which leads to Tc fluctuation. This study reveals the relationship between modifications made to the boron plane condition of MgB2 by the ZnO buffer layer and the superconducting Tc behavior of MgB2.