PHYSICAL REVIEW B 101, 155101 (2020) Doping-induced disorder and conductivity anisotropy in the spin density wave state of iron pnictides Dheeraj Kumar Singh 1, 2, 3 , * and Yunkyu Bang 2, 3, † 1 School of Physics and Materials Science, Thapar Institute of Engineering and Technology, Patiala 147004, Punjab, India 2 Department of Physics, POSTECH, Pohang, Gyeongbuk 790-784, Korea 3 Asia Pacific Center for Theoretical Physics, Pohang, Gyeongbuk 790-784, Korea (Received 12 January 2020; accepted 10 March 2020; published 1 April 2020) We examine the role of doped impurity atoms on the conductivity anisotropy of the spin-density wave state in iron pnictides. The conductivity is calculated in a self-consistent spin-density wave state with random impurities in a two-orbital model. We find that the increase in impurity concentration leads to an increased suppression of conductivity in both the antiferromagnetic and ferromagnetic directions. However, the conductivity anisotropy is larger in comparison to the Drude-weight anisotropy in the hole-doped regions. The difference between the two is explained in terms of the anisotropic scattering by the impurities. We demonstrate the effect of the anisotropic impurity scattering by calculating the modulation in the density of states introduced by a single impurity. It is also shown that an increase in the Drude-weight anisotropy with changing carrier concentration, which results mainly from the reconstructed band characteristics, may be directly linked to a similar anisotropy in the density-of-states modulation caused by the impurity atom. DOI: 10.1103/PhysRevB.101.155101 I. INTRODUCTION Anisotropic electronic properties in iron-based supercon- ductors have been a recurrent theme since the time they were discovered. While anisotropy is naturally expected in the (π, 0) spin-density wave (SDW) state with broken fourfold symmetry, its presence in the paramagnetic nematic as well as in the superconducting state has remained one of the long- standing issues. The signature of electronic anisotropy in the metallic SDW state is obtained through various experiments such as transport measurement [1–4], optical conductivity [5], angle-resolved photoemission spectroscopy (ARPES) [6,7], and scanning tunneling microscopy (STM) [8–14]. As revealed in the transport measurements, the direction with antiferromagnetic (AFM) spin arrangements is more conducting than the ferromagnetic (FM) direction, a behavior remarkably in contrast with what is expected according to the double-exchange mechanism [15]. The ratio of conductivities in the two directions can be as large as ≈2. The anisotropy continues to exist in the doped sample exhibiting the SDW state and also in the nematic phase without any long-range order [16]. Quasiparticle interference (QPI), probed by the STM mea- surements, can shed light on the nature of impurity scattering and hence on the conductivity anisotropy. Experiments show that the QPI patterns are highly anisotropic and appear nearly one dimensional in the SDW state, nematic phase, and su- perconducting state. The modulation in the local density of states (LDOS) is stronger along the AFM direction of the SDW state or along the a axis in the nematic phase with * dheeraj.kumar@thapar.edu † ykbang@apctp.org orthorhombic symmetry. Recent work suggests that the orbital splitting between the d xz and d yz orbitals may be crucial in explaining the quasi-one-dimensional nature of QPI patterns. An insight into the charge dynamics [5] was provided by the theoretical investigation of optical conductivity within the mean-field methods [17–19] as well as by a combined local- density approximation plus dynamical mean-field theory [20]. The in-plane anisotropy was traced to the orbital-weight dis- tribution along the reconstructed Fermi surfaces, which are elliptical in shape [21]. Transport properties in the SDW state have been studied using the memory-function approach [22] and methods based on semiclassical theory [23]. However, the origin of resistivity anisotropy as well as of the doping dependence remains controversial [24,25]. The roles of two important factors are highly debated, namely, the doping-induced (i) disorder and (ii) reconstructed band. An impurity may form elongated magnetic droplets that will enhance the anisotropy [23,26,27] while the interference between scatterings is also expected to play an important role [28]. The features associated with the band structure including the ellipticity of the electron pockets and the Dirac point in the vicinity of the Fermi level were emphasized. The roles of other factors such as critical spin fluctuations have been also investigated [29]. Despite the considerable progress in understanding the anisotropy in electronic properties, it is not clear how the Drude-weight anisotropy and the highly anisotropic QPI patterns are interlinked. Does an anisotropic QPI imply an anisotropy in the Drude weight or conductivity? The answer can provide an important insight into the role of more than one impurity distributed randomly. In this paper, we address the above questions. In particular, we examine the interplay between the roles of band structure and the doped impurity atoms in the conductivity anisotropy. We consider isotropic impurity scatterers, the presence of 2469-9950/2020/101(15)/155101(7) 155101-1 ©2020 American Physical Society