Magnetic and superconducting properties of Ca1−xNaxFe2As2 and FeSe as determined by μSR and NMR experiments

Muon Spin Rotation (μSR) and Nuclear Magnetic Resonance (NMR) are very important tools to study the microscopic properties of strongly correlated electrons system, in particular for the magnetic and superconducting properties of the unconventional superconductors. We studied, representatives of 122 and 11 Fe-pnictides, Ca1−xNaxFe2As2 and FeSe single crystals by means of μSR and NMR spectroscopy. CaFe2As2 is a semimetal, which exhibits spin density wave order below 167 K. By Ca→Na substitution, the magnetic order is suppressed and superconductivity emerges with Tc ≈ 34 K at optimal doping including a substitution level region where both phases coexist. We have investigated the interplay of order parameters in this coexistence region, and found nanoscopic coexistence of both order parameters. This is proven by a reduction of the magnetic order parameter by 7 % below the superconducting transition temperature. On the other hand, pure FeSe shows no magnetic ordering but superconducting transition at Tc= 9 K, and a structural transition at Ts= 90 K. Apart from interesting superconducting and magnetic properties, currently FeSe is on the focus of many researcher in the context of nematicity. To perceive this from a microscopic point of view, we studied high quality single crystals of FeSe by means of μSR and NMR technique. While we observed the presence of short-range quasi-static correlations in FeSe, the wave symmetry of the superconducting order parameter has an unusual field dependency.