Modelling colossal magnetoresistance manganites

Abstract

I briefly survey here attempts to model the rich and strange behaviour of colossal magnetoresistance manganites, after outlining some of the phenomena observed in them, and describing the three relevant strong local interactions of the e_g electrons (in two different orbital states at each site), namely with Jahn-Teller phonon modes (strength g), with resident t_2g spins (ferromagnetic Hund's rule coupling J_H) and amongst each other (the Mott Hubbard correlation U) . A new two fluid model of nearly localized l polarons and band (b) electrons for low energy behaviour emerges for large g; some of its applications are mentioned here. I describe some results of strong coupling U, J_H calculations in single site DMFT (Dynamical Mean Field Theory), and show that in the wide orbital liquid regime many characteristic manganite phenomena such as an insulating ferromagnetic ground state, thermal insulator metal transition, colossal magnetoresistance (cmr), materials systematics and the observed low effective carrier density can all be understood qualitatively and quantitatively. We also discuss the two 'phase' coexistence frequently found in these systems, and show that electrostatic coulomb interactions mute lb phase separation into nanoscale electronic inhomogeneity with l regions and b puddles. Finally, some problems of current interest as well as general ones arising, eg polarons and the physics of large electron phonon coupling g in the adiabatic regime, are mentioned.