Strong Competition of Superconducting and Magnetic Order Parameters in   Ba1-xNaxFe2As2
  We study the interplay of magnetic and superconducting order in single crystalline hole doped Ba1-xNaxFe2As2 using muon spin relaxation. We find microscopic coexistence of magnetic order and superconductivity. In a strongly underdoped specimen the two forms of order coexist without any measurable reduction of the ordered magnetic moment by superconductivity, while in a nearly optimally doped sample the ordered magnetic moment is strongly suppressed below the superconducting transition temperature. This coupling can be well described within the framework of an effective two-band model incorporating inter- and intra-band interactions. In optimally doped Ba1-xNaxFe2As2 we observe no traces of static or dynamic magnetism and the temperature dependence of the superfluid density is consistent with two s-wave gaps without nodes. 
Hemke Maeter, Gwendolyne Pascua, Hubertus Luetkens, Johannes Knolle,   Saicharan Aswartham, Sabine Wurmehl, G\"unter Behr, B...
cond-mat.supr-con