Observation of semi-localized dispersive states in the strongly   correlated electron-doped ferromagnet Eu$_{1-x}$Gd$_{x}$O
  Chemical substitution plays a key role in controlling the electronic and magnetic properties of complex materials. For instance, in EuO, carrier doping can induce a spin-polarized metallic state, colossal magnetoresistance, and significantly enhance the Curie temperature. Here, we employ a combination of molecular-beam epitaxy, angle-resolved photoemission spectroscopy, and an effective model calculation to investigate and understand how semi-localized states evolve in lightly electron doped Eu$_{1-x}$Gd$_{x}$O above the ferromagnetic Curie temperature. Our studies reveal a characteristic length scale for the spatial extent of the donor wavefunctions which remains constant as a function of doping, consistent with recent tunneling studies of doped EuO. Our work sheds light on the nature of the semiconductor-to-metal transition in Eu$_{1-x}$Gd$_{x}$O and should be generally applicable for doped complex oxides. 
Daniel E. Shai, Mark H. Fischer, Alex J. Melville, Eric J. Monkman,   John W. Harter, Dawei Shen, Darrell G. Schlom, Michael...
cond-mat.str-el