Interplay between spin density wave and superconductivity in '122' iron   pnictides: 57Fe M\"ossbauer study
  Iron-based superconductors Ba0.7Rb0.3Fe2As2 and CaFe1.92Co0.08As2 of the '122' family have been investigated by means of the 14.41-keV Moessbauer transition in 57Fe versus temperature ranging from the room temperature till 4.2 K. A comparison is made with the previously investigated parent compounds BaFe2As2 and CaFe2As2. It has been found that Moessbauer spectra of these superconductors are composed of the magnetically split component due to development of spin density wave (SDW) and non-magnetic component surviving even at lowest temperatures. The latter component is responsible for superconductivity. Hence, the superconductivity occurs in the part of the sample despite the sample is single phase. This phenomenon is caused by the slight variation of the dopant concentration across the sample (crystal). 
A. Blachowski, K. Ruebenbauer, J. Zukrowski, Z. Bukowski, M. Matusiak,   J. Karpinski
cond-mat.supr-con cond-mat.str-el