Charge ordering via electron-electron interactions in the colossal-magnetoresistive manganites

The coupled magnetic and charge-order transition observed in the manganites of the type R1-xMxMnO3 near half filling (x similar or equal to 1/2) is shown to be the result of the interplay between the double-exchange, superexchange, and the Coulomb interaction terms in an electronic Hamiltonian. At half tilling and temperature T=0 we find, as we increase the strength of the extended-Hubbard repulsion, a first-order transition from a charge-nonordered ferromagnetic metal (FN) to a charge-ordered antiferromagnetic and insulating (AFO) ground state. The AFO-FN transition is also obtained by increasing T: however, a small degree of charge order remains ill the ferromagnetic phase. The charge-ordered state also ''melts,'' as observed, on the application of a magnetic field, which causes a rapid drop in the transition temperature. Qualitative differences ill behavior between members of the manganite series can be understood in terms of small variations in the interaction parameters.