Comprehensive study of out-of-plane transport properties in BaFe2As2: Three-dimensional electronic state and effect of chemical substitution

We investigated the out-of-plane transport properties of parent and chemically substituted BaFe2As2 for various types of substitution. Based on the studies of the Hall coefficient and chemical-substitution effect, we have clarified the origin for the unusual temperature dependence of out-of-plane resistivity rho(c)(T) in the high-temperature paramagnetic-tetragonal phase. Electron (hole) carriers have an incoherent (coherent) character, which is responsible for nonmetallic (metallic) rho(c)(T). Although the electron and hole contributions are almost comparable, a slightly larger contribution comes from electrons at high temperatures but from holes at low temperatures, resulting in a maximum in rho(c)(T). In the low-temperature antiferromagnetic-orthorhombic phase, the major effect of substitution is to increase the residual-resistivity component, as in the case for the in-plane transport. In particular, Co atoms substituted for Fe give rise to strong scattering with large ac anisotropy. We found that K substitution induces a nonmetallic behavior in rho(c)(T) at low temperatures, which is likely due to a weakly localized nature along the c-axis direction.