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 ${\mathrm{BaFe}}₂{\mathrm{As}}₂$ 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 ${\ensuremath{\rho}}_c(T)$ in the high-temperature paramagnetic-tetragonal phase. Electron (hole) carriers have an incoherent (coherent) character, which is responsible for nonmetallic (metallic) ${\ensuremath{\rho}}_c(T)$. 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 ${\ensuremath{\rho}}_c(T)$. 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 ${\ensuremath{\rho}}_c(T)$ at low temperatures, which is likely due to a weakly localized nature along the $c$-axis direction.