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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 $T_s\approx 60$ K, forming an electronic nematic phase with two 90$^\circ$ rotated (twinned) domains, and orders antiferromagnetically below $T_N\approx 42$ 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 $E_\text{crossover} \approx 100$ meV are dominated by $d_{yz}$-$d_{yz}$ intra-orbital scattering processes, which have the two-fold ($C_2$) 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 $d_{xy}$-$d_{xy}$ intra-orbital scattering that controls the overall magnetic bandwidth in this material. In addition, we find that the low-energy ($E\approx 6$ meV) spin excitations change from approximate $C_4$ to $C_2$ rotational symmetry below a temperature $T^\ast$ ($>T_s$), 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 an uniaxial strain necessary to detwin NaFeAs also raises the onset temperature $T^\ast$ 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.