Competition/coexistence 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 (mu SR) 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, T-m similar or equal to T-c similar or equal to 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 T-m close to that of the parent compound (T-c << T-m similar or equal to 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 the superconducting phase, an assessment is made on the anisotropy of the 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 [n(s)(T)] measured by mu SR. The gap parameter, 2 Delta/k(B)T(c), determined from n(s)(T) exhibits a tendency that values in the hole-doped pnictides (Ba1-xKxFe2As2) are much greater than those in electron-doped ones (LaFeAsO1-xFx and CaFe1-xCoxAsF), suggesting a difference in the coupling to bosons mediating the Cooper pairs between relevant d electron bands.