Competition/Coexisitence of Magnetism and Superconductivity in Iron Pnictides Probed by Muon Spin Rotation

The presence of macroscopic phase separation into superconducting and magnetic phases in LaFeAsO1−xFx and CaFe1−xCoxAsF is demonstrated by muon spin rotation (μSR) measurement across their phase boundaries (x = 0.06 for LaFeAsO1−xFx and x = 0.075–0.15 for CaFe1−xCoxAsF). In LaFeAsO0.94F0.06, both magnetism and superconductivity develop simultaneously below a common critical temperature, Tm ≃ Tc ≃ 18 K, where the magnetism is characterized by strong randomness. A similar, but more distinct segregation of these two phases is observed in CaFe1−xCoxAsF, where the magnetic phase retains Tm as close to that of the parent compound (Tc ≪ Tm ≃ 80–120 K) and the superconducting volume fraction is mostly proportional to the Co content x. The close relationship between magnetism and superconductivity is discussed based on these experimental observations. Concerning superconducting phase, an assessment is made on the anisotropy of order parameter in the superconducting state of LaFeAsO1−xFx, CaFe1−xCoxAsF, and Ba1−xKxFe2As2 (x = 0.4) based on the temperature dependence of superfluid density [ns(T )] measured by μSR. The gap parameter, 2∆/kBTc, determined from ns(T ) exhibits a tendency that hole-doped pnictides (Ba1−xKxFe2As2) is much greater than those in electrondoped ones (LaFeAsO1−xFx, CaFe1−xCoxAsF), suggesting difference in the coupling to bosons mediating the Cooper pairs between relevant d electron bands. Magnetism and superconductivity in iron pnictides probed by μSR 2