Direct observation of the influence of the As-Fe-As angle on the Tc of superconducting SmFeAsO${}_{1-x}$F${}_x$

The electrical resistivity, crystalline structure and electronic properties calculated from the experimentally measured atomic positions of the compound SmFeAsO${}_{0.81}$F${}_{0.19}$ have been studied up to pressures ~20GPa. The correlation between the pressure dependence of the superconducting transition temperature (Tc) and crystallographic parameters on the same sample shows clearly that a regular FeAs${}_4$ tetrahedron maximizes Tc, through optimization of carrier transfer to the FeAs planes as indicated by the evolution of the electronic band structures.