Orbital order and spin nematicity in the tetragonal phase of the electron-doped iron pnictidesNaFe1−xCoxAs

In copper-oxide and iron-based high-temperature (high-${T}_{\mathrm{c}})$ 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 ${T}_{\mathrm{s}}$ coincides with the magnetic transition temperature ${T}_{\mathrm{N}}$, making the orbital and spin degrees of freedom highly entangled. NaFeAs is a system where ${T}_{\mathrm{s}}=54$ K is well separated from ${T}_{\mathrm{N}}=42$ K, which helps simplify the experimental situation. Here we report nuclear magnetic resonance (NMR) measurements on ${\mathrm{NaFe}}_{1\ensuremath{-}x}{\mathrm{Co}}_{x}\mathrm{As}$ $(0\ensuremath{\le}x\ensuremath{\le}0.042)$ that revealed orbital and spin nematicity occurring at a temperature ${T}^{*}$ far above ${T}_{\mathrm{s}}$ 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 ${T}_{\mathrm{s}}$, which brings about the observed spin nematicity.