[PCP1] Physics and Chemistry1

[PCP1-02] Local structure analysis of NdFeAs(O,H) and NdFeAs(O,F) thin films

*Takafumi Hatano¹, Koki Miyamoto¹, Shunya Tomioka¹, Atsuro Yoshikawa¹, Hiroshi Ikuta^1,2 (1. Dept. Materials Physics, Nagoya Univ. (Japan), 2. RCCME, Nagoya Univ. (Japan))

Keywords:

iron-based superconductor,thin film

The superconducting properties of thin films of iron-based superconductors (IBSs) often depend on the choice of substrate. Indeed, NdFeAs(O,F) (Nd1111:F) films grown on CaF₂ exhibit a T_c of 56 K, while those on MgO have a somewhat lower T_c, below ~45 K. Such substrate dependence has been attributed to strain-induced modification of local structurer parameters such as the As-Fe-As bond angle alpha. However, how the local structure is deformed by strain, and possible contributions from other factors, remain unclear. We examined the local structures of Nd1111:F and NdFeAs(O,H) (Nd1111:H) films grown on CaF₂ and MgO using X-ray absorption fine structure (XAFS) analysis. In Nd1111:H/MgO, alpha decreased monotonically with decreasing H substitution but remained above 110deg, whereas T_c was nearly constant (~40-45 K). In contrast, alpha of Nd1111F/CaF₂ was ~109.8deg, and T_c exceeded 50 K. These results are in line with the report that alpha = 109.47deg is optimal for the superconductivity in this system. However, we note that alpha of Nd1111F and Nd1111H seems to follow the same c-axis dependence, yet T_c does not. This suggest that there may be other factors that affect the superconducting properties of this system.