Interplay among spin, charge and the lattice in perovskite manganites

Perovskite manganites have received much attention due to the large magnetoresistance observed near the Curie Temperature {dollar}Tsb{lcub}C{rcub},{dollar} which marks the transition from a high-temperature paramagnetic insulator to a low-temperature ferromagnetic metal. We have studied 30% hole-doped LaMnO{dollar}sb3,{dollar} the doping range for optimal ferromagnetism, and found a rich phase diagram as a function of the average ionic radius of the La-site {dollar}langle rsb{lcub}A{rcub}rangle,{dollar} which is varied by substituting different rare earth ions for La. With decreasing {dollar}langle rsb{lcub}A{rcub}rangle, Tsb{lcub}C{rcub}{dollar} decreases rapidly and the magnitude of the magnetoresistance increases exponentially, until a zero temperature insulating phase is entered.; The structure of this phase diagram is consistent with changes is the electronic bandwidth induced by changes in the Mn-O bond lengths and the Mn-O-Mn bond angles, as measured by neutron diffraction. We find that the effects of applied pressure can be mapped onto changes in {dollar}langle rsb{lcub}A{rcub}rangle,{dollar} which can be thought of as a form of internal "chemical" pressure. In addition to structurally induced changes in the bandwidth, we demonstrate the important role of distortions of the MnO{dollar}sb6{dollar} octahedra, which increase with decreasing {dollar}langle rsb{lcub}A{rcub}rangle,{dollar} providing a tendency towards charge localization.; We have also examined the magnetoresistance in the low temperature ferromagnetic metallic state of single crystal and polycrystalline {dollar}rm Lasb{lcub}2/3{rcub}Srsb{lcub}1/3{rcub}MnOsb3.{dollar} We find that the intrinsic negative magnetoresistance in single crystal is due to the suppression of spin fluctuations, and magnetic domain boundaries do not dominate the scattering process. In contrast, in polycrystalline samples we find a large low-field magnetoresistance dominated by spin-polarized tunneling between grains.; Finally we present inelastic neutron scattering experiments on the spin dynamics in {dollar}rm Prsb{lcub}0.63{rcub}Srsb{lcub}0.37{rcub}MnOsb3.{dollar} Two distinct new features have been observed: a softening of the magnon dispersion at the zone boundary for {dollar}T