Poisoning effect of Mn in LaFe1-xMnxAsO0.89F0.11: Unveiling a quantum critical point in the phase diagram of iron-based superconductors

A superconducting-to-magnetic transition is reported for LaFeAsO0.89F0.11 where a per-thousand amount of Mn impurities is dispersed. By employing local spectroscopic techniques like muon spin rotation (μSR) and nuclear quadrupole resonance (NQR) on compounds with Mn contents ranging from x=0.025% to x=0.75%, we find that the electronic properties are extremely sensitive to the Mn impurities. In fact, a small amount of Mn as low as 0.2% suppresses superconductivity completely. Static magnetism, involving the FeAs planes, is observed to arise for x>0.1% and becomes further enhanced upon increasing Mn substitution. Also a progressive increase of low-energy spin fluctuations, leading to an enhancement of the NQR spin-lattice relaxation rate T−11, is observed upon Mn substitution. The analysis of T−11 for the sample closest to the crossover between superconductivity and magnetism (x=0.2%) points toward the presence of an antiferromagnetic quantum critical point around that doping level.