Enhancement of low-frequency fluctuations and superconductivity breakdown in Mn-doped La$_{1-y}$Y$_{y}$FeAsO$_{0.89}$F$_{0.11}$ superconductors
$^{19}$F NMR measurements in optimally electron-doped La$_{1-y}$Y$_{y}$Fe$_{1-x}$Mn$_{x}$AsO$_{0.89}$F$_{0.11}$ superconductors are presented. In these materials the effect of Mn doping on the superconducting phase is studied for two series of compounds ($y= 0$ and $y= 0.2$) where the chemical pressure is varied by substituting La with Y. In the $y=0.2$ series superconductivity is suppressed for Mn contents an order of magnitude larger than for the $y=0$ series. For both series a peak in the $^{19}$F NMR nuclear spin-lattice relaxation rate $1/T_1$ emerges upon Mn doping and gets significantly enhanced on approaching the quantum phase transition between the superconducting and magnetic phases. $^{19}$F NMR linewidth measurements show that for similar Mn contents magnetic correlations are more pronounced in the $y=0$ series, at variance with what one would expect for $\vec Q=(π/a,0)$ spin correlations. These observations suggest that Mn doping tends to reduce fluctuations at $\vec Q=(π/a,0)$ and to enhance other low-frequency modes. The effect of this transfer of spectral weight on the superconducting pairing is discussed along with the charge localization induced by Mn.