Control of the superconducting properties of Sr2−x Ca x VO3FeAs through isovalent substitution.

Abstract: The effect of the isovalent substitution of Sr2+ by Ca2+ on the structure and superconducting properties of Sr2−x Ca x VO3FeAs is described in the compositional range 0≤x≤0.5. SQUID magnetometry measurements reveal that after an initial increase in T c , which is maximised at 29.5K in Sr1.95Ca0.05VO3FeAs, a rapid suppression of superconductivity is observed with increasing x. XANES spectra of Sr2−x Ca x VO3FeAs collected on the Fe and V absorption K-edges show that the position of both edges are invariant with composition within the experimental uncertainty. A combination of synchrotron X-ray powder diffraction and neutron powder diffraction techniques is used to rationalise the observed changes in T c with x, in terms of changes to the structure of the FeAs layer upon partial Ca substitution. These findings demonstrate that superconductivity in the Fe-based superconductors is extremely sensitive to the crystal structure with T c maximised in samples with regular FeAs4-tetrahedra. [Copyright &y& Elsevier]