Ferromagnetism and electron-phonon coupling in the manganites

Abstract

The physics of ferromagnetic doped manganites, such as\n\\chem{La_{1-x}Ca_xMnO_3} with $x\\approx0.2$--0.4, is reviewed. The concept of\ndouble exchange is discussed within the general framework of itinerant electron\nmagnetism. The new feature in this context is the coupling of electrons to\nlocal phonon modes. Emphasis is placed on the quantum nature of the phonons and\nthe link with polaron physics. However it is stressed that the manganites fall\nin an intermediate coupling regime where standard small-polaron theory does not\napply. The recently-developed many-body coherent potential approximation is\nable to deal with this situation and Green's recent application to the Holstein\ndouble-exchange model is described. Issues addressed include the nature of the\nbasic electronic structure, the metal-insulator transition, a unification of\ncolossal magnetoresistance, pressure effects and the isotope effect, pseudogaps\nin spectroscopy and the effect of electron-phonon coupling on spin waves.\n