Electronic phase separation in the slightly underdoped iron pnictide   superconductor Ba(1-x)K(x)Fe(2)As(2)
  Here we present a combined study of the slightly underdoped novel pnictide superconductor Ba(1-x)K(x)Fe(2)As(2) by means of X-ray powder diffraction, neutron scattering, muon spin rotation (muSR), and magnetic force microscopy (MFM). Commensurate static magnetic order sets in below Tm ~ 70 K as inferred from the emergence of the magnetic (1 0 -3) reflection in the neutron scattering data and from the observation of damped oscillations in the zero-field-muSR asymmetry. Transverse-field muSR below Tc shows a coexistence of magnetically ordered and non-magnetic states, which is also confirmed by MFM imaging. We explain such coexistence by electronic phase separation into antiferromagnetic and superconducting/normal state regions on a lateral scale of several tens of nanometers. Our findings indicate that such mesoscopic phase separation can be considered an intrinsic property of some iron pnictide superconductors. 
J. T. Park, D. S. Inosov, Ch. Niedermayer, G. L. Sun, D. Haug, N. B.   Christensen, R. Dinnebier, A. V. Boris, A. J. Drew, L...