NMR Evidence for Inhomogeneous Nematic Fluctuations in ${\mathrm{BaFe}}_2({\mathrm{As}}_{1-x}{\mathrm{P}}_x{)}_2$

We present evidence for nuclear spin-lattice relaxation driven by glassy nematic fluctuations in isovalent P-doped ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ single crystals. Both the ${}^{75}\mathrm{As}$ and ${}^{31}\mathrm{P}$ sites exhibit a stretched-exponential relaxation similar to the electron-doped systems. By comparing the hyperfine fields and the relaxation rates at these sites we find that the As relaxation cannot be explained solely in terms of magnetic spin fluctuations. We demonstrate that nematic fluctuations couple to the As nuclear quadrupolar moment and can explain the excess relaxation. These results suggest that glassy nematic dynamics are a common phenomenon in the iron-based superconductors.

Figure 1

$(T_{1}T{)}^{-1}$ and $\beta$ for ${}^{75}\mathrm{As}$ (filled circle) and ${}^{31}\mathrm{P}$ (open square) vs temperature for $x=0.26$ (underdoped) and $x=0.33$ (optimal doping).

Figure 2

(a) Ratio of the spin-lattice relaxation rates of As to P (${{}^{75}T}_1^{-1}{/}^{31}{T}_1^{-1}$) and (b) $\mathrm{\Delta }(T_{1}T{)}^{-1}$ versus temperature for $x=0.26$ (filled circle) and $x=0.33$ (filled triangle). The gray horizontal region indicates the range of values for purely magnetic fluctuations. Inset: the theoretical ratio as a function of the antiferromagnetic correlation length $\xi$. The solid lines in (b) are best fits as discussed in the text.

Figure 3

Spectra of the central resonance of the ${}^{75}\mathrm{As}$ (a) and ${}^{31}\mathrm{P}$ (b) as a function of angle $\theta$ with respect to the $\widehat{c}$ axis for $x=0.26$. The markers represent the centers of the respective peaks extracted from global fits (solid black lines) to each of three data sets at 0°, 72°, and 90°. The dashed black lines show the calculated angular dependence of the resonance centers of the peaks based on extracted fit coefficients for the internal fields.