Anisotropy of the in-plane resistivity of underdoped Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ superconductors induced by impurity scattering in the antiferromagnetic orthorhombic phase

TL;DR

This study reveals that in underdoped Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ superconductors, in-plane resistivity anisotropy is primarily caused by impurity scattering from Co atoms in the antiferromagnetic orthorhombic phase, not Fermi surface anisotropy.

Contribution

It demonstrates that impurity scattering by Co atoms is the main cause of resistivity anisotropy, challenging previous explanations based on Fermi surface properties.

Findings

Resistivity anisotropy increases with Co concentration in the ordered phase.

Impurity scattering from Co atoms is anisotropic and proportional to doping level.

Impurity scattering becomes weak and isotropic in the paramagnetic-tetragonal phase.

Abstract

We investigated the in-plane resistivity anisotropy for underdoped Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ single crystals with improved quality. We demonstrate that the anisotropy in resistivity in the magnetostructural ordered phase arises from the anisotropy in the residual component which increases in proportion to the Co concentration $x$. This gives evidence that the anisotropy originates from the impurity scattering by Co atoms substituted for the Fe sites, rather than so far proposed mechanism such as the anisotropy of Fermi velocities of reconstructed Fermi surface pockets. As doping proceeds to the paramagnetic-tetragonal phase, a Co impurity transforms to a weak and isotropic scattering center.