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

TL;DR

This study models the interplay of orbital, lattice, and electronic interactions in perovskite manganese oxides, revealing how these factors influence charge dynamics, Jahn-Teller distortions, and phase separation, aligning with experimental observations.

Contribution

It introduces an extended double-exchange model incorporating Coulomb, orbital, and Jahn-Teller effects, and uses quantum Monte Carlo to analyze their roles in manganite properties.

Findings

Reproduces charge gap and Jahn-Teller distortion energies in undoped compounds.

Identifies critical enhancement of charge compressibility and orbital correlations upon doping.

Shows collapse of Jahn-Teller distortion and phase separation tendencies with doping.

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.