${}^{19}\mathrm{F}$ NMR measurements in optimally electron-doped ${\mathrm{La}}_{1\text{−}y}{\mathrm{Y}}_y{\mathrm{Fe}}_{1\text{−}x}{\mathrm{Mn}}_x{\mathrm{AsO}}_{0.89}{\mathrm{F}}_{0.11}$ 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 ${}^{19}\mathrm{F}$ NMR nuclear spin-lattice relaxation rate $1/T_1$ emerges upon Mn doping and becomes significantly enhanced on approaching the quantum phase transition between the superconducting and magnetic phases. ${}^{19}\mathrm{F}$ 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 $\vec{Q}=(\pi /a,0)$ spin correlations. These observations suggest that Mn doping tends to reduce fluctuations at $\vec{Q}=(\pi /a,0)$ 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.

• Received 12 April 2015
• Revised 10 June 2015

DOI:https://doi.org/10.1103/PhysRevB.92.020505