Ordering and Fluctuation of Orbital and Lattice Distortion in Perovskite Manganese Oxides

Abstract

Roles of orbital and lattice degrees of freedom in strongly correlated systems are investigated to understand electronic properties of perovskite Mn oxides such as La_{1-x}Sr_{x}MnO_{3}. An extended double-exchange model containing Coulomb interaction, doubly degenerate orbitals and Jahn-Teller coupling is derived under full polarization of spins with two-dimensional anisotropy. Quantum fluctuation effects of Coulomb interaction and orbital degrees of freedom are investigated by using the quantum Monte Carlo method. In undoped states, it is crucial to consider both the Coulomb interaction and the Jahn-Teller coupling in reproducing characteristic hierarchy of energy scales among charge, orbital-lattice and spin degrees of freedom in experiments. Our numerical results quantitatively reproduce the charge gap amplitude as well as the stabilization energy and the amplitude of the cooperative Jahn-Teller distortion in undoped compounds. Upon doping of carriers, in the absence of the Jahn-Teller distortion, critical enhancement of both charge compressibility and orbital correlation length is found with decreasing doping concentration. These are discussed as origins of strong incoherence in charge dynamics. With the Jahn-Teller coupling in the doped region, collapse of the Jahn-Teller distortion and instability to phase separation are obtained and favorably compared with experiments. These provide a possible way to understand the complicated properties of lightly doped manganites.

arXiv comment22 pages RevTeX including 25 PS figures, submitted to Phys.Rev.B, replaced version; two figures are replaced by Fig.17 with minor changes in the text

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• Colossal Magnetoresistant Materials: The Key Role of Phase Separation
• Charge-orbital ordering and phase-separation in the two-orbital model for manganites: Roles of Jahn-Teller phononic and Coulombic interactions
• Spin-orbital physics for p orbitals in alkali RO_2 hyperoxides --- generalization of the Goodenough-Kanamori rules
• Effects of Boson Dispersion in Fermion-Boson Coupled Systems
• Incoherent Charge Dynamics in Perovskite Manganese Oxides
• Dynamical mean field theory for manganites
• Correlation Effects on the Double Exchange Model in a Ferromagnetic Metallic Phase
• Phase diagram of a generalized Hubbard model applied to orbital order in manganites
• Microwave properties of Nd_0.5Sr_0.5MnO_3: a key role of the (x^2-y^2)-orbital effects