Impurity-Induced Electronic Nematic State in Iron-Pnictide Superconductors

We propose that impurity-induced electronic nematic state is realized above the orthorhombic structure transition temperature $T_S$ in iron-pnictide superconductors [1]. In the presence of strong orbital fluctuations near $T_S$, it is theoretically revealed that a single impurity induces non-local orbital order with $C_2$-symmetry, consistently with recent STM/STS measurements. Each impurity-induced $C_2$ orbital order aligns along a-axis by applying tiny uniaxial pressure along b-axis. In this impurity-induced nematic phase, the resistivity shows sizable in-plane anisotropy ($\rho_b/\rho_a \sim 2$) even above $T_S$, actually observed in various ``detwinned" samples. The present study indicates the existence of strong orbital fluctuations in iron-pnictide superconductors. [1] Y. Inoue, Y. Yamakawa and H. Kontani, arXiv:1110.2401.