Orbital Order, Stripe Phases and Mott Transition in a Planar Model for Manganites

TL;DR

This paper investigates orbital ordered Mott insulating states in manganites, revealing their connection to stripe phases and the Mott transition, using DMFT and diagonalisation methods to match experimental observations.

Contribution

It provides an exact theoretical description of orbital ordered states and stripe phases in a planar manganite model, aligning with experimental data.

Findings

Derived ground states match experimental observations for specific doping levels.

Identified orbital ordering as an alloy-like arrangement of orbitals.

Connected orbital order to stripe phases and the Mott transition.

Abstract

Understanding orbital ordered (OO) Mott insulating states lies at the heart of a consistent resolution of the colossal magneto-resistance (CMR) observed in manganites, where its melting induces a low-$T$ insulator-metal transition for $0.25 \le x\le 0.45$. Motivated thereby, we study the OO states in a planar model for bilayer manganites using DMFT and finite-size diagonalisation methods. We derive the correct OO ground states observed in manganites for $x=0,{1/2},{2/3},{3/4}$ in exact agreement with observations, including the charge-orbital-magnetic ordered stripe phases for $x>{1/2}$. These OO states are {\it exactly} shown to be associated with an ``alloy'' ordering of the $d_{3x^{2}-r^{2}}/d_{3y^{2}-r^{2}}$ orbitals on each $Mn^{3+}$ site.