Simultaneous Optimization of Spin Fluctuations and Superconductivity under Pressure in an Iron-Based Superconductor

We present a high-pressure NMR study of the overdoped iron pnictide superconductor ${\mathrm{NaFe}}_{0.94}{\mathrm{Co}}_{0.06}\mathrm{As}$. The low-energy antiferromagnetic spin fluctuations in the normal state, manifest as the Curie-Weiss upturn in the spin-lattice relaxation rate $1/{}^{75}T_{1}T$, first increase strongly with pressure but fall again at $P>P_{\mathrm{opt}}=2.2\mathrm{GPa}$. Neither long-ranged magnetic order nor a structural phase transition is encountered up to 2.5 GPa. The superconducting transition temperature $T_c$ shows a pressure dependence identical to the spin fluctuations. Our observations demonstrate that magnetic correlations and superconductivity are optimized simultaneously as a function of the electronic structure, thereby supporting very strongly a magnetic origin of superconductivity.

Figure 1

Main panel: RF resonance frequency of the detuned NMR circuit measured as a function of temperature and pressure at zero field. The onset and midpoint superconducting transition temperatures, respectively $T_c^{\mathrm{onset}}$ and $T_c^m$, are indicated by the arrows. Inset: values of $T_c^{\mathrm{onset}}$ and $T_c^m$ as functions of pressure.

Figure 2

(a) Center line of the ${}^{75}\mathrm{As}$ NMR spectra at different pressures, with field applied in the crystalline $ab$ plane. (b) Pressure dependence of the ${}^{75}\mathrm{As}$ quadrupole frequency ${}^{75}\nu _q$ at $T=30\mathrm{K}$. (c) Temperature dependence of the Knight shift ${}^{75}K_{ab}$ at different pressures.

Figure 3

(a) Temperature dependence of $1/{}^{75}T_{1}T$ at different pressures. The solid lines are fits to the form $1/{}^{75}T_{1}T=A_1/(T-\theta )+B_{1}T+C_1{T}^2$. (b) Pressure dependence of the Curie-Weiss temperature $\theta$ extracted from panel (a). (c) Comparison of $1/{}^{75}T_{1}T$ data near $T_c$ at the two highest pressures [data and fitting lines as in panel (a)].

Figure 4

Main panel: midpoint superconducting transition temperature $T_c^m$ (squares) and normal-state spin-lattice relaxation rate $1/{}^{75}T_{1}T$ at $T=30\mathrm{K}$ (diamonds) as a function of pressure. Inset: scaling between $T_c$ and normal-state $1/{}^{75}T_{1}T$.