Enhancement of low-frequency fluctuations and superconductivity breakdown in Mn-doped La1-y YyFeAsO0.89 F0.11 superconductors

19F NMR measurements in optimally electron-doped La1-yYyFe1-xMnxAsO0.89F0.11 superconductors are presented. 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 y = 0. For both series a peak in the 19FNMRnuclear spin-lattice relaxation rate 1/T1 emerges upon Mn doping and becomes significantly enhanced on approaching the quantum phase transition between the superconducting and magnetic phases. 19F 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 Q = (π/a,0) spin correlations. These observations suggest that Mn doping tends to reduce fluctuations at 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.