${}^{31}\text{P}$ and ${}^{75}\text{A}\text{s}$ NMR evidence for a residual density of states at zero energy in superconducting ${\text{BaFe}}_2{\left({\text{As}}_{0.67}{\text{P}}_{0.33}\right)}_2$

${}^{31}\text{P}$ and ${}^{75}\text{A}\text{s}$ NMR measurements were performed in superconducting ${\text{BaFe}}_2{\left({\text{As}}_{0.67}{\text{P}}_{0.33}\right)}_2$ with $T_c=30\mathrm{K}$. The nuclear-spin-lattice relaxation rate $T_1^{-1}$ and the Knight shift in the normal state indicate the development of antiferromagnetic fluctuations, and $T_1^{-1}$ in the superconducting (SC) state decreases without a coherence peak just below $T_c$, as observed in $\left({\text{Ba}}_{1-x}{\text{K}}_x\right){\text{Fe}}_2{\text{As}}_2$. In contrast to other iron arsenide superconductors, the $T_1^{-1}\propto T$ behavior is observed below 4 K, indicating the presence of a residual density of states at zero energy. Our results suggest that strikingly different SC gaps appear in ${\text{BaFe}}_2{\left({\text{As}}_{1-x}{\text{P}}_x\right)}_2$ despite a comparable $T_c$ value, an analogous phase diagram, and similar Fermi surfaces to $\left({\text{Ba}}_{1-x}{\text{K}}_x\right){\text{Fe}}_2{\text{As}}_2$.

Figure 1

(Color online) ac susceptibility of ${\text{BaFe}}_2{\left({\text{As}}_{0.67}{\text{P}}_{0.33}\right)}_2$ obtained with an NMR coil at $\sim 17\text{MHz}$, indicating $T_c\left(H=0\right)=30\text{K}$. Inset: schematic phase diagram of ${\text{BaFe}}_2{\left({\text{As}}_{1-x}{\text{P}}_x\right)}_2$. $T_0$ $\left(T_{\text{AFM}}\right)$ represents the structural (AFM) ordering temperature. The arrow denotes the P substitution $x$ of the present sample with maximum $T_c$.

Figure 2

(Color online) ${}^{31}\text{P}$ Knight shift $K$ for different magnetic fields. Inset shows ${}^{31}\text{P}$ NMR spectra in 4.12 T. The spectra have been offset vertically.

Figure 3

(Color online) ${}^{31}\text{P}$ (right axis) and ${}^{75}\text{A}\text{s}$ (left axis) ${\left(T_{1}T\right)}^{-1}$ for ${\text{BaFe}}_2{\left({\text{As}}_{0.67}{\text{P}}_{0.33}\right)}_2$, along with ${}^{75}\text{A}\text{s}$ ${\left(T_{1}T\right)}^{-1}$ in optimally doped iron arsenide superconductors (Refs. 2, 5, 10, 12, 14, 23).

Figure 4

(Color online) (a) and (b) show ${}^{31}\text{P}$ nuclear-spin relaxation curves for 175 and 1.5 K in 4.12 T, exhibiting clear single exponential behavior. (c) ${}^{31}\text{P}$ $T_1^{-1}$ under 2.1, 4.12, and 6.0 T. Inset in (c) shows ${}^{31}\text{P}$ $T_1^{-1}$ vs magnetic field at 2 K; a linear fit is used to extrapolate $T_1^{-1}$ for $H\to 0\text{T}$.

Figure 5

(Color online) Normalized ${}^{31}\text{P}$ $T_1^{-1}$ for ${\text{BaFe}}_2{\left({\text{As}}_{0.67}{\text{P}}_{0.33}\right)}_2$ and $\left({\text{Ba}}_{0.6}{\text{K}}_{0.4}\right){\text{Fe}}_2{\text{As}}_2$ (Ref. 12). The solid curve shows a calculation for a line-node gap function $\Delta \left(\varphi \right)={\Delta }_0\text{sin}\left(2\varphi \right)$ with $2{\Delta }_0=6k_B{T}_c$ and $N_{\text{res}}/N_0=0.34$.