Local spin fluctuations in iron-based superconductors: 77Se and 87Rb NMR measurements of Tl0.47Rb0.34Fe1.63Se2

We report nuclear magnetic resonance (NMR) studies of the intercalated iron selenide superconductor (Tl, Rb)${}_y$Fe${}_{2-x}$Se${}_2$ ($T_c=32$ K). Single-crystal measurements up to 480 K on both ${}^{77}$Se and ${}^{87}$Rb nuclei show a superconducting phase with no magnetic order. The Knight shifts $K$ and relaxation rates $1\mathrm{/}{T}_{1}T$ increase very strongly with temperature above $T_c$, before flattening at 400 K. The quadratic $T$-dependence and perfect proportionality of both $K$ and $1\mathrm{/}{T}_{1}T$ data demonstrate their origin in paramagnetic moments. A minimal model for this pseudogap-like response is not a missing density of states but two additive contributions from the itinerant electronic and local magnetic components, a framework unifying the $K$ and $1\mathrm{/}{T}_{1}T$ data in many iron-based superconductors.