Orbital order and spin nematicity in the tetragonal phase of the electron-doped iron pnictides NaFe1-xCoxAs

In copper-oxide and iron-based high-temperature (high-T-c) superconductors, many physical properties exhibit in-plane anisotropy, which is believed to be caused by a rotational symmetry-breaking nematic order, whose origin and its relationship to superconductivity remain elusive. In many iron pnictides, a tetragonal-to-orthorhombic structural transition temperature T-s coincides with the magnetic transition temperature T-N, making the orbital and spin degrees of freedom highly entangled. NaFeAs is a system where T-s = 54 K is well separated from T-N = 42 K, which helps simplify the experimental situation. Here we report nuclear magnetic resonance (NMR) measurements on NaFe1-xCoxAs (0 <= x <= 0.042) that revealed orbital and spin nematicity occurring at a temperature T* far above T-s in the tetragonal phase. We show that the NMR spectra splitting and its evolution can be explained by an incommensurate orbital order that sets in below T* and becomes commensurate below T-s, which brings about the observed spin nematicity.