As Single Crystals*

We present our ²³Na and ⁷⁵As NMR study of the over-doped pnictide NaFe_1-xCu_xAs, which has been demonstrated to be a possible Mott insulator near x ≈ 0.5 [1,2]. Our NMR spectral-weight and linewidth analysis of the ²³Na quadrupolar spectrum reveals inequivalent Na sites and indicates a progressive formation of real space Cu and Fe stripe ordering as the Cu concentration approaches 0.5. Our spin-lattice relaxation data shows an antiferromagnetic transition at 190 K for x = 0.48. At lower Cu concentration there is a spin-glass transition evident from both susceptibility and spin-lattice relaxation data that appears at 80 K for x = 0.39. We have performed numerical simulation of our ⁷⁵As lineshape by testing a Cu-induced staggered magnetization model and discuss these in the context of the ²³Na NMR data. [1] Song, Yu, et al., Nat. Commun. 7, 13879 (2016). [2] C.E. Matt et al.Phys. Rev. Lett. 117, 097001 (2016)