Figure 1

The SDW and superconducting (SC) phase transition temperatures, $T_{\mathrm{SDW}}$ and $T_c$, observed for our samples. Also shown in the inset are the schematic representations of the Fermi surface geometry in the unfolded first Brillouin zone: the underdoped and optimally doped regimes $x<0.15$ (left), and the overdoped nonsuperconducting regime (right). Dashed and solid arrows represent the intraband and interband transitions, respectively. Filling of the hole Fermi surface by doped electrons results in the absence of ${\mathbf{Q}}_{\mathrm{AF}}\sim (\pi /a,0)$ interband transitions between the hole and electron Fermi surfaces in the overdoped regime.Reuse & Permissions

Figure 2

Field-swept ${}^{75}\mathrm{As}$ NMR line shapes of overdoped $\mathrm{Ba}({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x{)}_2{\mathrm{As}}_2$ with $x=0.09$ at 20 K ($>T_c$), $x=0.14$ at 4.2 K ($>T_c$), and nonsuperconducting $x=0.26$ at 4.2 K with external magnetic field $B\parallel c$ axis. The NMR frequency is fixed at $f=56.555\mathrm{MHz}$. The gray dashed line marks the expected resonance position for $K=0$. Notice the systematic increase in the relative intensity of the As(N) peaks ($N=1–3$) for larger $x$. Inset: schematic representations of the Fe and Co coordinations of As(0) and As(1) sites.Reuse & Permissions

Figure 3

$\mathbf{q}=\mathbf{0}$ uniform susceptibility as measured by ${}^{75}\mathrm{As}$ NMR Knight shift $K$ for As(0) sites in representative compositions. The dashed curve is a fit of $x=0.26$ data to an activation form, $K=0.20+0.23\times \exp (-\Delta /T)$, with $\Delta /k_B=450\mathrm{K}$. Inset: $(1/T_{1}T{)}^{0.5}$ and $1/T_{1}T$ for $x=0.26$ plotted as a function of $K$ with temperature as the implicit parameter.Reuse & Permissions

Figure 4

$1/T_{1}T$ measured at As(0) sites for various concentrations $x$ with magnetic field $B$ applied along the $ab$ plane. Solid and dashed arrows mark $T_c$ and $T_{\mathrm{SDW}}$, respectively. Solid and dashed curves are the best fits with (for $x\le 0.14$) and without (for $x=0.26$) a Curie-Weiss term arising from AFSF; see the main text for details.Reuse & Permissions

Figure 5

The concentration dependence of (a) the strength of paramagnetic spin fluctuations as measured by $1/T_{1}T$ observed at 25 K, and (b) Weiss temperature $\theta$ obtained from the fit in Fig. 4. Solid curves are guides for the eyes.Reuse & Permissions