Orbital polarons versus itinerant e_g electrons in doped manganites

TL;DR

This paper investigates how orbital polarons form and evolve in doped manganites, revealing a transition from localized polarons to metallic phases influenced by doping, temperature, and Jahn-Teller effects.

Contribution

It introduces a detailed 1D orbital t-J model for doped manganites, analyzing the crossover from localized polarons to metallic phases and the effects of temperature and Jahn-Teller potential.

Findings

Localized orbital polarons form at low doping.

Doping induces phase separation and metallic phases.

Jahn-Teller potential stabilizes orbital order and enhances localization.

Abstract

We study an effective one-dimensional (1D) orbital t-J model derived for strongly correlated e_g electrons in doped manganites. The ferromagnetic spin order at half filling is supported by orbital superexchange prop. to J which stabilizes orbital order with alternating x^2-y^2 and 3z^2-r^2 orbitals. In a doped system it competes with the kinetic energy prop. to t. When a single hole is doped to a half-filled chain, its motion is hindered and a localized orbital polaron is formed. An increasing doping generates either separated polarons or phase separation into hole-rich and hole-poor regions, and eventually polarizes the orbitals and gives a it metallic phase with occupied 3z^2-r^2 orbitals. This crossover, investigated by exact diagonalization at zero temperature, is demonstrated both by the behavior of correlation functions and by spectral properties, showing that the orbital chain with Ising superexchange is more classical and thus radically different from the 1D spin t-J model. At finite temperature we derive and investigate an effective 1D orbital model using a combination of exact diagonalization with classical Monte-Carlo for spin correlations. A competition between the antiferromagnetic and ferromagnetic spin order was established at half filling, and localized polarons were found for antiferromagnetic interactions at low hole doping. Finally, we clarify that the Jahn-Teller alternating potential stabilizes the orbital order with staggered orbitals, inducing the ferromagnetic spin order and enhancing the localized features in the excitation spectra. Implications of these findings for colossal magnetoresistance manganites are discussed.