Momentum-Space Dichotomy in the Metal-Insulator Transition in doped EuO

EuO possesses a wide variety of remarkable properties, most which can be accessed only upon carrier doping. In addition to its large ferromagnetic moment (S = 7/2), doped EuO exhibits a metal-insulator transition with a change in resistivity of over $10^{13}$ and highly spin polarized carriers. Furthermore, the ferromagnetic Curie temperature can be enhanced from 69 K in undoped EuO to over 200 K in carrier doped EuO. We present angle-resolved photoemission studies of Eu$_{1-x}$Gd$_x$O thin films which elucidate the electronic structure and mechanism of the metal-insulator transition. Our ARPES studies verify that the exchange coupling between the Eu 4f moments and the delocalized Eu 5d states pushes the bottom of the majority-spin conduction band through $E_F$ below $T_C$. We also reveal a surprising dichotomy between the delocalized carriers at the Brillouin zone boundary below $T_C$, and localized carriers around the zone center above $T_C$ which are responsible for the respective low-temperature ferromagnetic metallic and high-temperature paramagnetic semiconducting behaviors observed in transport measurements.