Charge order and the metal-insulator transitions in A(1/2)A ' 1/2MnO3

An interpretation is given for the metal-insulator transitions occurring in A (1/2)A(1/2)(')MnO(3) using the double-exchange model but including the c oupling of the electron to the Jahn-Teller distortions and a finite Coulomb interaction between electrons in the twofold degenerate e(g) orbitals. We find that, whereas the ferromagnetic state is metallic, the insulating stat e is polaronic. As one lowers the temperature, a transition from the ferrom agnetic state to the CE phase is observed, which is, due to the strong phon on renormalization of the bandwidth, accompanied by a change in resistivity of several orders of magnitude. In order to obtain the observed magnetic s tructure, i.e., CE type, it is essential to include the lowering of the on- site energy due to polaronic self-energy effects. We present a way of build ing the charge-ordered magnetic structures out of small units of a "localiz ed" 3z(2)-r(2) polaron with four antiferromagnetic and two ferromagnetic ne ighboring t(2g) core spins. [S0163-1829(99)00923-6].