  The discovery of high-temperature superconductivity in iron pnictides raised
the possibility of an unconventional superconducting mechanism in multiband
materials. The observation of Fermi-surface(FS)-dependent nodeless
superconducting gaps suggested that inter-FS interactions may play a crucial
role in superconducting pairing. In the optimally hole-doped
Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$, the pairing strength is enhanced
simultaneously (2$\Delta$/Tc$\sim$7) on the nearly nested FS pockets, i.e. the
inner holelike ($\alpha$) FS and the two hybridized electronlike FSs, while the
pairing remains weak (2$\Delta$/Tc$\sim$3.6) in the poorly-nested outer
hole-like ($\beta$) FS. Here we report that in the electron-doped
BaFe$_{1.85}$Co$_{0.15}$As$_2$ the FS nesting condition switches from the
$\alpha$ to the $\beta$ FS due to the opposite size changes for hole- and
electron-like FSs upon electron doping. The strong pairing strength
(2$\Delta$/Tc$\sim$6) is also found to switch to the nested $\beta$ FS,
indicating an intimate connection between FS nesting and superconducting
pairing, and strongly supporting the inter-FS pairing mechanism in the
iron-based superconductors.
