Anisotropic impurity states, quasiparticle scattering and nematic transport in underdoped Ca(Fe1−xCox)2As2

When CaFe 2 As 2 is lightly doped with Co an electronic liquid-crystalline state emerges, which becomes the ‘parent’ state of high-temperature superconductivity in this ferropnictide. A spectroscopic imaging study shows that the ‘nematic’ order is likely to be an artefact of the doping itself. Iron-based high-temperature superconductivity develops when the ‘parent’ antiferromagnetic/orthorhombic phase is suppressed, typically by introduction of dopant atoms 1 . But their impact on atomic-scale electronic structure, although in theory rather complex 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , is unknown experimentally. What is known is that a strong transport anisotropy 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 with its resistivity maximum along the crystal b axis 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , develops with increasing concentration of dopant atoms 14 , 20 , 21 , 22 , 23 , 24 , 25 ; this ‘nematicity’vanishes when the parent phase disappears near the maximum superconducting T c . The interplay between the electronic structure surrounding each dopant atom, quasiparticle scattering therefrom and the transport nematicity has therefore become a pivotal focus 7 , 8 , 12 , 22 , 23 of research into these materials. Here, by directly visualizing the atomic-scale electronic structure, we show that substituting Co for Fe atoms in underdoped Ca(Fe 1− x Co x ) 2 As 2 generates a dense population of identical anisotropic impurity states. Each is ∼ 8 Fe–Fe unit cells in length, and all are distributed randomly but aligned with the antiferromagnetic a axis. By imaging their surrounding interference patterns, we further demonstrate that these impurity states scatter quasiparticles in a highly anisotropic manner, with the maximum scattering rate concentrated along the b axis. These data provide direct support for the recent proposals 7 , 8 , 12 , 22 , 23 that it is primarily anisotropic scattering by dopant-induced impurity states that generates the transport nematicity; they also yield simple explanations for the enhancement of the nematicity proportional to the dopant density 14 , 20 , 21 , 22 , 23 , 24 , 25 and for the occurrence of the highest resistivity along the b axis 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 .