Slow tail of nematic spin fluctuations in Ba(Fe1-xCox)2As2: Insight from nuclear magnetic resonance

The two most interesting properties of Fe-based superconductors (FeSCs), high-temperature superconductivity and nematicity, are widely believed to come from spin fluctuations (SF). One of the least studied aspects of SF are their frequency spectrum, albeit it is generally believed that SF in FeSCs can be roughly separated into (1) fast SF in the spin space which preserve the local nematic correlations, (2) slow SF in the real space, where the sense of the local nematic order fluctuates, and (3) the high-energy SF which break the local nematic order. Although, the latter are probably irrelevant in the entire observable temperature range. Nuclear quadrupolar interactions, as opposed to magnetic interactions, are sensitive only to SF in the real space, and only to those slower than its characteristic timescale. We combine existing nuclear magnetic resonance data on electric field gradient anisotropy with first-principles calculations to access the amplitude of the real-space (nematic) SF on the timescale above the actual nematic transition in BaFe2As2 as a function of Co doping. This demonstrates exceptionally slow dynamics of nematic fluctuations even in the formally tetragonal phase.