LAUSR

A central question in a large class of strongly correlated electron systems, including heavy fermion compounds and iron pnictides, is the identification of different phases and their origins. It has been shown that the antiferromagnetic (AFM) phase in some heavy fermion compounds is induced by Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between localized moments, and that the competition between this interaction and Kondo effect is responsible for quantum criticality. However, conclusive experimental evidence of the RKKY interaction in pnictides is lacking. Here, using high resolution Na NMR measurements on lightly Cu-doped metallic single crystals of NaFe1−xCuxAs (x ≈ 0.01), we show direct evidence of the RKKY interaction in this pnictide system. Aided by computer simulation, we identify the Na NMR satellite resonances with the RKKY oscillations of spin polarization at Fe sites. Our NMR results indicate that magnetic order is due to both electronic itinerancy and local moments in this compound.