Two Ferromagnetic States in Magnetoresistive Manganites - First Order Transition Driven by Orbitals -

TL;DR

This paper investigates how orbital degrees of freedom influence the metal-insulator transition in manganites, revealing a first-order phase transition between ferromagnetic metallic and insulating states driven by orbital ordering.

Contribution

It introduces a derived effective Hamiltonian considering orbital degeneracy and electron correlations, demonstrating a first-order transition linked to orbital order-disorder.

Findings

First-order phase transition between ferromagnetic metal and insulator.

Orbital order-disorder transition observed via X-ray scattering.

Orbital degrees of freedom play a crucial role in metal-insulator transition.

Abstract

A systematic study of the electronic structure in perovskite manganites is presented. The effective Hamiltonian is derived by taking into account the degeneracy of $e_g$ orbitals and strong electron correlation in Mn ions. The spin and orbital orderings are examined as functions of carrier concentration in the mean-field approximation applied to the effective Hamiltonian. We obtain the first order phase transition between ferromagnetic metallic and ferromagnetic insulating states in the lightly doped region. The transition is accompanied with the orbital order-disorder one which is directly observed in the anomalous X-ray scattering experiments. The present investigation shows a novel role of the orbital degree of freedom on metal-insulator transition in manganites.