Phase Diagram in Manganese Oxides

TL;DR

This paper theoretically explores the complex phase diagram of manganites considering orbital degeneracy, revealing various magnetic and orbital phases influenced by doping, interactions, and distortions, aligning with recent experimental observations.

Contribution

It introduces a detailed mean field theoretical model accounting for orbital degeneracy and Jahn-Teller effects in manganites, expanding understanding of their phase behavior.

Findings

Identification of a metallic A-type antiferromagnetic state at x~0.5 with orbital order

Demonstration of orbital polarization controlling conduction band dimensionality

Correlation between doping, spin, and orbital structures consistent with experiments

Abstract

We study theoretically the phase diagram of perovskite manganites taking into account the double degeneracy of the $e_g$ orbitals in a $Mn^{3+}$ ion. A rich phase diagram is obtained in the mean field theory at zero temperature as functions of $x$ (hole concentration) and $J_S$ (antiferromagnetic interaction between $t_{2g}$ spins). The global features of the phase diagram is understood in terms of the superexchange and double exchange interactions, which are strongly depends on types of the occupied $e_g$ orbitals. The strong electron correlation induces the orbital polarization, which controls the dimension of the conduction band. A sequential change of the spin and orbital structures with doping holes is consistent with the recent experiments. In particular, metallic A-type (layered) antiferromagnetic state is found for $x\sim0.5$ with the uniform $d_{x^2-y^2}$ orbital ordering. Effects of the Jahn-Teller distortion are also studied. A short version of this paper has been already published (to appear in PRB-rapid June), but this paper contains additional and more detailed results.