Orbital selective neutron spin resonance in underdoped superconducting NaFe$_\textbf{0.985}$Co$_\textbf{0.015}$As

We use neutron scattering to study the electron-doped superconducting NaFe$_{0.985}$Co$_{0.015}$As ($T_c=14$ K), which has co-existing static antiferromagnetic (AF) order ($T_N=31$ K) and exhibits two neutron spin resonances ($E_{r1}\approx 3.5$ meV and $E_{r2}\approx 6$ meV) at the in-plane AF ordering wave vector ${\bf Q}_{\rm AF}={\bf Q}_{1}=(1,0)$ in reciprocal space. In the twinned state below the tetragonal-to-orthorhombic structural transition $T_s$, both resonance modes appear at ${\bf Q}_{1}$ but cannot be distinguished from ${\bf Q}_{2}=(0,1)$. By detwinning the single crystal with uniaxial pressure along the orthorhombic $b$-axis, we find that both resonances appear only at ${\bf Q}_{1}$ with vanishing intensity at ${\bf Q}_{2}$. Since electronic bands of the orbital $d_{xz}$ and $d_{yz}$ characters split below $T_s$ with the $d_{xz}$ band sinking $\sim10$ meV below the Fermi surface, our results indicate that the neutron spin resonances in NaFe$_{0.985}$Co$_{0.015}$As arise mostly from quasi-particle excitations between the hole and electron Fermi surfaces with the $d_{yz}$ orbital character.