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Recent neutron scattering measurements indicate that NaFe 1−x Cu x As 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 using 23 Na nuclear magnetic resonance (NMR) spectra and spinlattice 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 pseudo gap 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 Cu 2+ to a Cu 1+ state as x exceeds 0.18, concomitant with the change of Fe 2+ to Fe 3+ resulting in the formation of magnetic clusters. However, for x ≤ 0.39 there is no evidence of long-range static magnetic order.

Neutron scattering measurements have demonstrated that the heavily Cu-doped NaFe1−xCuxAs compound behaves like a Mott insulator exhibiting both real space Fe-Cu stripes, as well as antiferromagnetism below a Néel temperature for x < ∼ 0.5. We have investigated evolution of structural and magnetic ordering using 23 Na and 75 As NMR for single crystals (x = 0.39 and 0.48), confirming antiferromagnetism in the form of magnetic stripes. We show that end-chain defects in these stripes are the principal source of magnetic disorder and are responsible for cluster spin-glass transitions in both compounds, in the latter case coexistent with antiferromagnetism. Aided by our numerical simulation of the 75 As spectra, we show that a staggered magnetization at the Fe sites is induced by non-magnetic Cu dopants.

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Charge order in cuprate superconductors appears to be a universal characteristic, often associated with pseudogap behavior in the normal state. The central question is whether such charge ordering or the pseudogap are required for the existence of high temperature superconductivity and embody its mechanism. An important but phenomenological approach to this question is to examine whether these phenomena extend over various members of the cuprate family. Recent nuclear magnetic resonance (NMR) measurements on oxygen chain-ordered single crystals of YBa 2 Cu 3 O 6+y (Y123) have demonstrated temperature and magnetic field induced charge ordering that was confirmed in x-ray experiments. In the present work on high-quality single crystals of the tetragonal compound, HgBa 2 CuO 4+δ , we use 17 O NMR to investigate the interplay between charge and spin order deduced from the full quadrupolar-split NMR spectrum over a wide range of temperature and magnetic field. We have found evidence for a coherent modulation of charge and spin order in this compound. However, neither temperature nor magnetic field induced ordering was observed and we infer that this aspect of high temperature superconductivity is not universal.