Workshop "NMR, µSR, Mössbauer spectroscopies in the study of Fe-based and other unconventional high-Tc superconductors"
The poisoning effect of Mn in LaFe1-xMnxAsO0.89F0.11: Unveiling a quantum critical point in the phase diagram of iron-based superconductors
Franziska Hammerath, TU Dresden, Germany
Local probe investigations on the extraordinary poisoning effect of Mn on superconductivity in LaFe1-xMnxAsO0.89F0.11  will be presented. Muon spin rotation (µSR) measurements show that the electronic properties are extremely sensitive to the Mn impurities, even though charge doping effects are negligible. Already 0.2% Mn suppresses superconductivity completely, while static magnetism, involving the FeAs planes, is observed for x = 0.1% Mn and becomes further enhanced upon increasing the Mn substitution . A progressive increase of low energy spin fluctuations, expressed in an enhanced 75As nuclear quadrupole resonance (NQR) spin-lattice relaxation rate 1/T1 is observed upon Mn substitution. The analysis of 1/T1 for the sample closest to the crossover between superconductivity and magnetism points towards an antiferromagnetic quantum critical point at this crossover . These results will be discussed in light of recent theoretical developments reporting on the impact of magnetic impurities in iron-based superconductors [3, 4]. Recent experimental results on the competition between the effects of Mn-doping and pressure and accordingly Mn-doping and Y for La substitution will complete the talk.
 M. Sato et al., J. Phys. Soc. Jpn. 79, 014710 (2010)
 F. Hammerath et al., Phys. Rev. B 89, 134503 (2014)
 R. M. Fernandes, A. J. Millis, Phys. Rev. Lett. 110, 117004 (2013)
 M. N. Gastiasoro, B. M. Andersen, arXiv:1403.3324 (2014)