Identical spin fluctuations in Cu- and Co-dopedBaFe2As2independent of electron doping

Abstract

We present $^{75}\mathrm{As}$ nuclear magnetic resonance measurements on single crystals of $\mathrm{Ba}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$, $\mathrm{Ba}{\mathrm{Fe}}_{1.8}{\mathrm{Co}}_{0.2}{\mathrm{As}}_{2}$, and $\mathrm{Ba}{\mathrm{Fe}}_{1.82}{\mathrm{Cu}}_{0.18}{\mathrm{As}}_{2}$. While only Co doping induces bulk superconductivity on a broad doping range, the spin fluctuations probed by the nuclear spin-lattice relaxation rate ${({T}_{1}T)}^{\ensuremath{-}1}$ are identical for both dopings down to ${T}_{c}$. Below this temperature, ${({T}_{1}T)}^{\ensuremath{-}1}$ of the Cu-doped sample continues to rise, proving that (a) there is a quantum critical point below the superconducting dome, and (b) adding electrons does not affect the spin fluctuations. Consequently, we analyze the Knight shift data in terms of a two-component scenario, with one hyperfine coupling to an itinerant degree of freedom and the other to Fe moments.