Competing effects of Mn and Y doping on the low-energy excitations and phase diagram of La${}_{1-y}$Y${}_y$Fe${}_{1-x}$Mn${}_x$AsO${}_{0.89}$F${}_{0.11}$ iron-based superconductors

Abstract

Muon Spin Rotation ($\mu$SR) and ${}^{19}$F Nuclear Magnetic Resonance (NMR) measurements were performed to investigate the effect of Mn for Fe substitutions in La${}_{1-y}$Y${}_y$Fe${}_{1-x}$Mn${}_x$AsO${}_{0.89}$F${}_{0.11}$ superconductors. While for $y=0$ a very low critical concentration of Mn ($x=0.2$%) is needed to quench superconductivity, as $y$ increases the negative chemical pressure introduced by Y for La substitution stabilizes superconductivity and for $y=20$% it is suppressed at Mn contents an order of magnitude larger. A magnetic phase arises once superconductivity is suppressed both for $y$=0 and for $y=20$%. Low-energy spin fluctuations give rise to a peak in ${}^{19}$F NMR $1/T_1$ with an onset well above the superconducting transition temperature and whose magnitude increases with $x$. Also the static magnetic correlations probed by ${}^{19}$F NMR linewidth measurements show a marked increase with Mn content. The disruption of superconductivity and the onset of the magnetic ground-state are discussed in the light of the proximity of LaFeAsO${}_{0.89}$F${}_{0.11}$ to a quantum critical point.

8 pages, 9 figures