Resistivity Anisotropy of $AE$Fe$_2$As$_2$ ($AE$ =Ca, Sr, Ba): direct versus Montgomery technique measurements

TL;DR

This study measures the resistivity anisotropy in AEFe2As2 compounds using direct and Montgomery techniques, revealing lower anisotropy than previously reported and a decrease across the structural transition, indicating a significant three-dimensional electronic structure.

Contribution

It provides a direct comparison of resistivity anisotropy measurements using two methods and clarifies the anisotropy magnitude and its temperature evolution in AEFe2As2 compounds.

Findings

Resistivity anisotropy is below 10 for all compounds.

Anisotropy increases with larger AE ions at room temperature.

Anisotropy decreases below the structural transition temperature.

Abstract

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 \gamma_\rho=\rho_c/\rho_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 \rho_a as compared with \rho_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.