Orbital selective neutron spin resonance in underdoped superconducting NaFe0.985Co0.015As

We use neutron scattering to study the electron-doped superconducting ${\mathrm{NaFe}}_0.985{\mathrm{Co}}_0.015\mathrm{As}$ (${T}_c=14$ K), which has coexisting static antiferromagnetic (AF) order (${T}_N=31$ K) and exhibits two neutron spin resonances (${E}_r1\ensuremath{\approx}3.5$ meV and ${E}_r2\ensuremath{\approx}6$ meV) at the in-plane AF ordering wave vector ${\mathbf{Q}}_{\mathrm{AF}}={\mathbf{Q}}₁=(1,0)$ in reciprocal space. In the twinned state below the tetragonal-to-orthorhombic structural transition ${T}_s$, both resonance modes appear at ${\mathbf{Q}}₁$ but cannot be distinguished from ${\mathbf{Q}}₂=(0,1)$. By detwinning the single crystal with uniaxial pressure along the orthorhombic $b$ axis, we find that both resonances appear only at ${\mathbf{Q}}₁$ with vanishing intensity at ${\mathbf{Q}}₂$. Since electronic bands of the orbital ${d}_xz$ and ${d}_yz$ characters split below ${T}_s$ with the ${d}_xz$ band sinking $\ensuremath{\sim}10$ meV below the Fermi surface, our results indicate that the neutron spin resonances in ${\mathrm{NaFe}}_0.985{\mathrm{Co}}_0.015\mathrm{As}$ arise mostly from quasiparticle excitations between the hole and electron Fermi surfaces with the ${d}_yz$ orbital character.