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

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 ($E_{r1}\approx 3.5$ meV and $E_{r2}\approx 6$ 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 $\sim 10$ 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.

Figure 1

(a) The phase diagram of ${\mathrm{NaFe}}_{1-x}{\mathrm{Co}}_x\mathrm{As}$ with the arrow indicating the Co-doping concentration in our experiment. The inset shows positions of magnetic excitations in the $[H,K]$ plane under uniaxial pressure along the $b$ axis. (b) Schematic Fermi surfaces of ${\mathrm{NaFe}}_{0.985}{\mathrm{Co}}_{0.015}\mathrm{As}$ in the paramagnetic tetragonal state and different orbitals are characterized with different colors. The arrows mark nesting wave vectors ${\mathbf{Q}}_{\mathbf{1}}$ = (1,0) and ${\mathbf{Q}}_{\mathbf{2}}$ = (0,1). (c) Schematic $d_{yz}$ and $d_{xz}$ orbital bands in ${\mathrm{NaFe}}_{1-x}{\mathrm{Co}}_x\mathrm{As}$ above and below $T_s$ as seen by ARPES [31, 32].

Figure 2

(a) Temperature differences of transverse scans at the $(1,0,0.5)$ and $(0,1,0.5)$ magnetic Bragg peak positions in ${\mathrm{NaFe}}_{0.985}{\mathrm{Co}}_{0.015}\mathrm{As}$ single crystal under uniaxial pressure of $\sim 10$ MPa. (b) Temperature dependence of the AF peak intensity at ${\mathbf{Q}}_1=(1,0,0.5)$ under uniaxial pressure with vertical dashed lines indicating $T_c=14$ K, $T_N=31$ K, and $T_s=40$ K. (c) Neutron spin resonance modes at ${\mathbf{Q}}_1=(1,0,0.5)$ and ${\mathbf{Q}}_2=(0,1,0.5)$ are obtained by taking temperature differences of the constant-$\mathbf{Q}$ scans below and above $T_c$ in uniaxial pressure partially detwinned sample. (d) Neutron spin resonance modes in pressure released a sample obtained using the same sample and the same sample holder with the same spectrometer setup as in (c). The similar intensity of the resonance at ${\mathbf{Q}}_1$ and ${\mathbf{Q}}_1$ indicate that the sample becomes nearly 100% twinned.

Figure 3

(a) Sum of neutron spin resonance mode intensities at ${\mathbf{Q}}_1$ and ${\mathbf{Q}}_2$ in the partially detwinned and pressure released sample separately. (b) The ratio between the temperature difference of constant-$\mathbf{Q}$ scans (below and above $T_c$) at ${\mathbf{Q}}_1$ and ${\mathbf{Q}}_2$ in the partially detwinned sample. Points with large error bars around $E=4.5$ meV are not shown. The open circle labels the detwinning ratio measured from the static AF order. (c) Temperature dependence of background subtracted spin excitations at $E_{r2}=6.5$ meV at ${\mathbf{Q}}_1$ and ${\mathbf{Q}}_2$ in the partially detwinned sample [45]. (d) The corresponding ratio of temperature dependence of spin excitations at $E_{r2}=6.5$ meV between ${\mathbf{Q}}_1$ and ${\mathbf{Q}}_2$. $T_c, T_N$, and $T^{*}>T_s$ are labeled with dashed lines in (c) and (d).

Figure 4

Temperature differences of constant energy scans at ${\mathbf{Q}}_1$ and ${\mathbf{Q}}_2$ between 1.5 and 21 K at (a) $E_{r1}=3.75$ meV, (b) $E_{r2}=6.5$ meV in the partially detwinned sample, and (c) $E_{r2}=6.5$ meV in the pressure released sample. (d) Comparison of the sum of the magnetic scattering at ${\mathbf{Q}}_1$ and ${\mathbf{Q}}_2$ for the partially detwinned and twinned sample.