Systemic Study of Orbital and Spin Nematicity in NaFe_1−xCo_xAs by NMR

Nematic order, a self-organized state with rotational symmetry broken, has been observed in both copper-oxide and iron-pnictide high temperature superconductors. However, its origin is still a mystery in the iron pnictides although it is considered as a key to understand the mechanism of superconductivity. Here, we report a systemic nuclear magnetic resonance (NMR) study on NaFe_1−xCo_xAs (0 ≤x ≤ 0.042) that an orbital order, accompanied by an instant spin nematicity, occurs at at a temperature T^∗ far above structural transition temperature T_s in the tetragonal phase. We show that the observed NMR spectra splitting and its evolution is due to an incommensurate orbital order that sets in below T^∗ and becomes commensurate below T_s. We show that the electric field gradient asymmetry parameter is a good measure for the orbital order parameter which undergoes a Landau-like 2nd-order phase transition. We further show that the spin nematicity is well accounted for by the observed orbital order.