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 $1/T_1T$ increase very strongly with temperature above $T_c$, before flattening at 400 K. The quadratic $T$-dependence and perfect proportionality of both $K$ and $1/T_1T$ 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 $1/T_1 T$ data in many iron-based superconductors.