Colossal magnetoresistance manganites: A new approach

Manganites of the LA1–xCaxMnO3 family show a variety of new and poorly understood electronic, magnetic and structural effects. Here we outline a new approach recently proposed by us, where we argue that due to strong Jahn–Teller (JT) coupling with phonons the twofold degenerate eg states at the Mn sites dynamically reorganize themselves into localised, JT polarons l with exponentially small inter-site hopping, and band-like, nonpolaronic states b, leading to a new 2-band model for manganites which includes strong Coulomb and Hund’s couplings. We also discuss some results from a dynamical mean-field theory treatment of the model which explains quantitatively a wide variety of experimental results, including insulator– metal transitions and CMR, in terms of the influence of physical conditions on the relative energies and occupation of the l and b states. We argue that this microscopic coexistence of the two types of electronic states, and their relative occupation and spatial correlation is the key to manganite physics.