Systemic Study of Orbital and Spin Nematicity in NaFe1−x Co x As 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 NaFe1−x Co x As (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.
KeywordsIron pnictides Nuclear magnetic resonance Nematic order
This work was done in collaboration with L.Y. Xing, X. C. Wang, and C. Q. Jin of Institute of Physics. We thank S. Maeda and T. Oguchi for advice and help in the EFG calculation, Z. Li and J. Yang for assistance in some of the measurements. This work was partially supported by CAS Strategic Priority Research Program, No. XDB07020200 and by a 973 project National Basic Research Program of China, No. 2012CB821402.