Orbital selective spin waves in detwinned NaFeAs

The existence of orbital-dependent electronic correlations has been recognized as an essential ingredient to describe the physics of iron-based superconductors. NaFeAs, a parent compound of iron-based superconductors, exhibits a tetragonal-to-orthorhombic lattice distortion below ${T}_s\ensuremath{\approx}60$ K, forming an electronic nematic phase with two ${90}^{\ensuremath{\circ}}$ rotated (twinned) domains, and orders antiferromagnetically below ${T}_N\ensuremath{\approx}42$ K. We use inelastic neutron scattering to study spin waves in uniaxial pressure-detwinned NaFeAs. By comparing the data with combined density functional theory and dynamical mean-field theory calculations, we conclude that spin waves up to an energy scale of ${E}_{\text{crossover}}\ensuremath{\approx}100$ meV are dominated by ${d}_yz\text{\ensuremath{-}}{d}_yz$ intraorbital scattering processes, which have the twofold (${C}₂$) rotational symmetry of the underlying lattice. On the other hand, the spin wave excitations above ${E}_{\text{crossover}}$, which have approximately fourfold (${C}₄$) rotational symmetry, arise from the ${d}_xy\text{\ensuremath{-}}{d}_xy$ intraorbital scattering that controls the overall magnetic bandwidth in this material. In addition, we find that the low-energy ($E\ensuremath{\approx}6$ meV) spin excitations change from approximate ${C}₄$ to ${C}₂$ rotational symmetry below a temperature ${T}^*$ ($>{T}_s$), while spin excitations at energies above ${E}_{\text{crossover}}$ have approximate ${C}₄$ rotational symmetry and are weakly temperature dependent. These results are consistent with angle-resolved photoemission spectroscopy measurements, where the presence of a uniaxial strain necessary to detwin NaFeAs also raises the onset temperature ${T}^*$ of observable orbital-dependent band splitting to above ${T}_s$, thus supporting the notion of orbital selective spin waves in the nematic phase of iron-based superconductors.