$s_{\pm }$-like spin resonance in the iron-based nodal superconductor BaFe${}_2$(As${}_{0.65}$P${}_{0.35}$)${}_2$ observed using inelastic neutron scattering

We performed inelastic neutron scattering on a powder sample of iron-based superconductor BaFe${}_2$(As${}_{0.65}$P${}_{0.35}$)${}_2$ with $T_c=30$ K. Although this system is expected to have line nodes in its superconducting order parameter, we have observed spin resonance at the same scattering vector as that appearing in the $s_{\pm }$-wave iron-based superconductors without line nodes. Moreover, the resonance enhancement, which can be a measure of the area of sign reversal between the hole and electron Fermi surfaces (FSs), is comparable to those without line nodes. These facts indicate that the sign reversal between the FSs is still dominant in this system, and the line nodes should create only a limited area of sign reversal on a single FS. Hence the system can hold higher $T_c$ than the other iron-based superconductors with nodal symmetry, such as LaFePO${}_{1-y}$ and KFe${}_2$As${}_2$ ($T_c<10$ K). Comparison with theoretical calculation indicates horizontal line nodes may be a candidate for reproducing the observation.

Figure 1

Dynamical structure factor $S(Q,E)$ of BaFe${}_2$(As${}_{0.65}$P${}_{0.35}$)${}_2$ measured with $E_i=45.5$ meV at (a) 5 K (below $T_c$) and (b) 32 K (above $T_c$). (c) and (d) show constant energy cuts of $S(Q,E)$ at 5 and 32 K, respectively, measured with $E_i=45.5$ meV. (e) and (f) are those measured with $E_i=21.6$ meV. Solid lines are the results of fits to a Gaussian function on a sloped background. The offset values of (c) and (d) are 6, 12, 20, 30, 40, 51, and 64 for 10–16 meV, and 5, 10, 15, and 20 for 6–9 meV of cuts (e) and (f) for clarity.

Figure 2

Temperature dependence of magnetic excitation peak height observed at $Q=1.2$ Å${}^{-1}$ and $E=12$ meV. Circles and diamonds represent data measured by TAS-1 and 4SEASONS, respectively. The solid line is a guide to the eye.

Figure 3

Energy dependence of $Q$-integrated dynamical spin susceptibility, ${\chi }^{\prime \prime }\left(E\right)$, around $Q_{\text{AF}}\sim 1.2$ Å${}^{-1}$ at 5 (circles) and 32 K (diamonds). Data measured with $E_i=45.5$ meV (filled symbols) and $E_i=21.6$ meV (unfilled symbols) are plotted together.

Figure 4

(a) Dynamical spin susceptibility ${\chi }^{\prime \prime }\left(E\right)$ obtained by the multiorbital RPA with use of the ten-orbital three-dimensional model at $Q\sim (1/21/21/4)$. Energy is scaled by the gap size (${\Delta }_0=6$ meV). (b)–(d) show schematic figures of horizontal line node, $s_{\pm }$-wave, and $d_{xy}$-wave model, respectively.