Theory of colossal magnetoresistive perovskite manganites

In this dissertation, the interrelationship of electron motion, magnetic order, and lattice distortion in perovskite manganites is studied theoretically.; First, a model of localized classical electrons coupled to lattice degrees of freedom and to each other via the Coulomb interaction is studied to gain insight into the charge and orbital ordering observed in lightly doped perovskite manganites. Explicit expressions are obtained for the minimum energy and ionic displacements caused by given hole and electron orbital configurations. These expressions are analyzed for several hole configurations, including the one experimentally observed by Yamada et al. in La_7/8Sr _1/8MnO₃. We find that, although the preferred charge and orbital ordering depends sensitively on parameters, there are ranges of the parameters in which the experimentally observed hole configuration has the lowest energy. For these parameter values we also find that the energy differences between different hole configurations are of the order of the observed charge ordering transition temperature. The effects of additional strains are also studied.; Second, we analyze the optical properties and spin wave stiffness of the 30% hole doped manganites, Nd_0.7Sr_0.3MnO₃, La_0.7Ca_0.3MnO₃, and La_0.7Sr _0.3MnO₃. Experimentally observed variation of the optical spectral weight with temperature is explained in terms of the transition between the Jahn-Teller small polaron state and the coherent transport in ferromagnetic state. A model of double exchange interaction and a tight-binding fit to the band structure are used to obtain the spin wave stiffness, which is found to be close to the observed value.; Third, a tight binding parameterization of the band structure, along with a mean field treatment of the Hund, electron-electron, and electron-lattice couplings, is used to obtain the full optical conductivity tensor of LaMnO ₃ as a function of temperature. We predict striking changes with temperature in the functional form and magnitude of the optical absorption. Comparison of our results with existing data make it possible to determine the electron-lattice and electron-electron couplings. The effective “Hubbard U” is found to be ≈1.6 eV, rather less than the full band width ≈3.6 eV, placing the material in the weak-intermediate coupling regime.; Fourth, the effects of uniaxial strain on the structural, orbital, optical, and magnetic properties of LaMnO₃ are calculated using a general elastic energy expression, along with a tight-binding parameterization of the band theory. Reasons why the observed (ππO) orbital ordering is favored over a (πππ) periodicity are discussed. Tensile uniaxial strain of the order of 2% (i.e., of the order of magnitude of those induced in thin films by lattice mismatch with substrates) can change the magnetic ground state, leading to dramatic changes in the band structure and optical conductivity spectrum. The magnetostriction effect associated with the Neel transition of bulk(unstrained) LaMnO₃ is also determined.