Local spin fluctuations in iron-based superconductors:Se77andRb87NMR measurements on Tl0.47Rb0.34Fe1.63Se2

Abstract

We report nuclear magnetic resonance (NMR) studies of the intercalated iron selenide superconductor ${(\text{Tl},\text{Rb})}_{y}{\text{Fe}}_{2\ensuremath{-}x}{\text{Se}}_{2}$ (${T}_{c}=32$ 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 $1/{T}_{1}T$ increase very strongly with temperature above ${T}_{c}$, before flattening at 400 K. The quadratic $T$-dependence and perfect proportionality of both $K$ and $1/{T}_{1}T$ data demonstrate their origin in paramagnetic moments. A minimal model for this pseudogap-type response is not a missing density of states, but rather 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.