Abstract
The spin dynamics in single crystal, electron-doped $\text{Ba}{({\text{Fe}}_{1\ensuremath{-}x}{\text{Co}}_{x})}_{2}{\text{As}}_{2}$ has been investigated by inelastic neutron scattering over the full range from undoped to the overdoped regime. We observe damped magnetic fluctuations in the normal state of the optimally doped compound $(x=0.06)$ that share a remarkable similarity with those in the paramagnetic state of the parent compound $(x=0)$. In the overdoped superconducting compound $(x=0.14)$, magnetic excitations show a gaplike behavior, possibly related to a topological change in the hole Fermi surface (Lifshitz transition) while the imaginary part of the spin susceptibility ${\ensuremath{\chi}}^{\ensuremath{''}}$ prominently resembles that of the overdoped cuprates. For the heavily overdoped, nonsuperconducting compound $(x=0.24)$ the magnetic scattering disappears, which could be attributed to the absence of a hole Fermi-surface pocket observed by photoemission.
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