Xin, YizhouStolt, IngridLee, Jeongseop A.Song, YuDai, PengchengHalperin, W.P.2019-08-132019-08-132019Xin, Yizhou, Stolt, Ingrid, Lee, Jeongseop A., et al.. "Toward the Mott state with magnetic cluster formation in heavily Cu-doped NaFe1−xCuxAs." <i>Physical Review B,</i> 99, no. 15 (2019) American Physical Society: https://doi.org/10.1103/PhysRevB.99.155114.https://hdl.handle.net/1911/106233Recent neutron-scattering measurements indicate that NaFe1−xCuxAs forms an antiferromagnetic stripe phase near x≈0.5 in a Mott insulating state. This copper concentration is well in excess of that required for superconductivity, x<0.04. We have investigated the development of magnetism in this compound by using 23Na nuclear magnetic resonance (NMR) spectra and spin-lattice relaxation measurements performed on single crystals (x=0.13, 0.18, 0.24, and 0.39). We find multiple inequivalent Na sites, each of which is associated with a different number of nearest-neighbor Fe sites occupied by a Cu dopant. We show that the distribution of Cu substituted for Fe is random in-plane for low concentrations (x=0.13 and 0.18) but deviates from this with increasing Cu doping. As is characteristic of many pnictide compounds, there is a spin pseudogap that increases in magnitude with dopant concentration. This is correlated with a corresponding increase in orbital NMR frequency shift, indicating a change in valence from Cu2+ to a Cu1+ state as x exceeds 0.18, concomitant with the change of Fe2+ to Fe3+ resulting in the formation of magnetic clusters. However, for x≤0.39 there is no evidence of long-range static magnetic order.engArticle is made available in accordance with the publisher's policy and may be subject to US copyright law. Please refer to the publisher's site for terms of use.Journal articlehttps://doi.org/10.1103/PhysRevB.99.155114