Induced lattice strain in epitaxial Fe-based superconducting films on CaF_2 substrates: A comparative study of the microstructures of SmFeAs(O,F), Ba(Fe,Co)_2As_2, and FeTe_(0.5)Se_(0.5)

摘要

The microstructures of SmFeAs(O,F), Ba(Fe,Co)_2As_2, and FeTe_(0.5)Se_(0.5) prepared on CaF_2 substrates were investigated using transmission electron microscopy. The SmFeAs(O,F)/CaF_2 interface is steep, without a disordered layer. By contrast, a chemical reaction occurs at the interface in the cases of Ba(Fe,Co)_2As_2 and FeTe_(0.5)Se_(0.5). The reaction layers are located on opposite sides of the interface for Ba(Fe,Co)_2As_2 and FeTe_(0.5)Se_(0.5). We found that the lattice distortion of the three superconducting films on the CaF_2 substrates enhances the T_C values compared with films prepared on oxide substrates. The origin of this lattice deformation varies depending on the superconducting material.