Resistivity anisotropy of A Fe2 As2 (A=Ca, Sr, Ba): Direct versus Montgomery technique measurements

The anisotropy of electrical resistivity was measured in parent compounds of the iron-arsenic high-temperature superconductors A Fe2 As2 with alkali earth elements A=Ca, Sr, and Ba. Measurements were performed using both the Montgomery technique and direct resistivity measurements on samples cut along principal crystallographic directions. The anisotropy ratio γρ = ρc / ρa is well below ten for all compounds in the whole temperature range studied (4-300 K), in notable contrast to previous reports. The anisotropy at room temperature increases from about two in Ca to about four in Sr and Ba. In all compounds the resistivity ratio decreases on cooling through the structural/antiferromagnetic transition temperature TSM, with the change mainly coming from stronger variation in ρa as compared with ρc. This suggests that the transition affects stronger the two-dimensional parts of the Fermi surface. We compare our experimental observations with band-structure calculations, and find similar trend in the evolution of anisotropy with the size of A ion. Our results show that the electronic structure of the iron pnictides has large contribution from three-dimensional areas of the Fermi surface.