Modelling colossal magnetoresistance manganites

Abstract

Rare-earth (Re) manganites (with alkaline-earth (Ak) ions partially substituting them),i.e. Re1−xAkxMnO3, have been intensively explored for the last decade or more because of the promiseof magnetoelectronic applications as well as because of complex and unusualphenomena in which electronic, structural and magnetic effects are intertwined. A briefsurvey of these and a description of the three strong local interactions of theeg electrons (in two different orbital states at each site), namely with Jahn–Teller phonon modes (strengthg), with residentt2g spins (ferromagneticHund’s rule coupling JH) and between each other (the Mott–Hubbard correlationU) form the background against which efforts at modelling manganitebehaviour are described. A new two-fluid model of nearly localized(l) polaronsand band (b) electrons for low-energy behaviour is hypothesized for largeg; some of its applications are mentioned here. First I describe some results of largeU,JH calculations in single-site DMFT (dynamical mean field theory) whichincludes the effect of all the strong local correlations. These results aredirectly appropriate for the orbital liquid regime, found typically for0.2<x<0.5, and not too low temperatures. I show that many characteristic manganite phenomena,such as an insulating ferromagnetic ground state, thermal insulator–metal transition(nearly coincident with the paramagnetic to ferromagnetic transition), colossalmagnetoresistance (CMR), materials systematics dependent on the specific Re and Ak ions,and the observed low effective carrier density, can all be understood qualitatively aswell as quantitatively. We also discuss the two-‘phase’ coexistence frequentlyfound in these systems, and show that electrostatic Coulomb interactions mutelb phase separation into nanoscale electronic inhomogeneity withl regionsand b puddles. Finally, some problems of current interest as well as general onesarising, e.g. polarons and the physics of large electron phonon couplingg in the adiabatic regime, are mentioned.