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Resistivity Anisotropy of $AE$Fe$_2$As$_2$ ($AE$ =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, AEFe2As2 with Alkali Earth elements AE=Ca,Sr, Ba. Measurements were performed using both the Montgomery technique and direct resistivity measurements on samples cut along principal crystallographic directions. The anisotropy ratio γ_ρ=ρ_c/ρ_a is well below 10 for all compounds in the whole temperature range studied (4 to 300 K), in notable contrast to previous reports. The anisotropy at room temperature increases from about 2 in Ca, to about 4 in Sr and Ba. In all compounds the resistivity ratio decreases on cooling through the structural/antiferromagnetic transition temperature T_{SM}, with the change mainly coming from stronger variation in ρ_a as compared with ρ_c. This suggests that the transitions stronger affect 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 AE ion. Our results show that the electronic structure of the iron pnictides has large contribution from three-dimensional areas of the Fermi surface.