Effects of Metallic Spacer in Layered Superconducting Sr-2(MgyTi1-y)O3FeAs

Abstract

The highly two-dimensional superconducting system Sr-2(MgyTi1-y)O3FeAs, recently synthesized in the range of 0.2 <= y <= 0.5, shows an Mg concentration-dependent T-c. Reducing the Mg concentration from y = 0.5 leads to a sudden increase in T-c, with a maximum T-c approximate to 40 K at y = 0.2. Using first principles calculations, the unsynthesized stoichiometric y = 0 and the substoichiometric y = 0.5 compounds have been investigated. For the 50 % Mg-doped phase (y = 0.5), Sr-2(MgyTi1-y)O-3 layers are completely insulating spacers between FeAs layers, leading to the fermiology such as that found for other Fe pnictides. At y = 0, representing a phase with metallic Sr2TiO3 layers, the Gamma-centered Fe-derived Fermi surfaces (FSs) considerably shrink or disappear. Instead, three Gamma-centered Ti FSs appear, and in particular two of them have similar size, like in MgB2. Interestingly, FSs have very low Fermi velocity in large fractions: the lowest being 0.6 x 10(6) cm/s. Furthermore, our fixed spin moment calculations suggest the possibility of magnetic ordering, with magnetic Ti and nearly nonmagnetic Fe ions. These results indicate a crucial role of Sr-2(MgyTi1-y)O-3 layers in this superconductivity.