Existence of a novel metallic ferromagnetic phase in models for undoped manganites

TL;DR

This paper demonstrates a novel metallic ferromagnetic phase in undoped manganite models, revealing a metal-insulator transition influenced by electron-phonon and Coulomb interactions, with implications for colossal magnetoresistance.

Contribution

It introduces a new metallic ferromagnetic phase in the $e_g$-orbital Hubbard model, contrasting with cuprate models, and explores its implications for manganite properties.

Findings

Metallic phase exists at finite electron-phonon and Coulomb interactions.

Fermi-surface topology prevents orbital ordering at half-filling.

Potential link to colossal magnetoresistive effects in manganites.

Abstract

The existence of a novel metal-insulator transition in the ferromagnetic state of models for undoped manganites is here discussed using numerical techniques applied to the $e_{\rm g}$-orbital degenerate Hubbard model tightly coupled with Jahn-Teller distortions. The ground-state phase diagram is presented in the plane defined by the electron-phonon coupling $\lambda$ and Coulomb interaction $u$. In contrast to the standard one-band Hubbard model for cuprates, the metallic phase is found to exist for finite values of both $\lambda$ and $u$ in the present $e_{\rm g}$-orbital Hubbard model even at half-filling, due to the Fermi-surface topology which is incompatible with the staggered orbital ordering concomitant to the insulating phase. Based on the present results, a possible scenario for Colossal Magneto-Resistive effect is discussed in undoped manganites.