Fragility of the A-type AF and CE Phases of Manganites: An Exotic Insulator-to-Metal Transition Induced by Quenched Disorder

TL;DR

This paper demonstrates that quenched disorder can induce a rapid insulator-to-metal transition in manganites, revealing the fragility of certain insulating phases and suggesting a new class of giant effects in complex oxides.

Contribution

The study provides a detailed analysis of how quenched disorder destabilizes insulating phases in manganites, leading to a disorder-induced insulator-metal transition, a phenomenon not previously characterized.

Findings

Disorder causes a rapid insulator-to-metal transition in manganites.

Fragile insulating states are destabilized by quenched disorder.

The transition mechanism involves percolative effects and proximity to ferromagnetic metallic phases.

Abstract

Using Monte Carlo simulations and the two eg-orbital model for manganites, the stability of the CE and A-type antiferromagnetic insulating states is analyzed when quenched disorder in the superexchange JAF between the t2g localized spins and in the on-site energies is introduced. At vanishing or small values of the electron-(Jahn-Teller)phonon coupling, the previously hinted "fragility" of these insulating states is studied in detail, focusing on their charge transport properties. This fragility is here found to induce a rapid transition from the insulator to a (poor) metallic state upon the introduction of disorder. A possible qualitative explanation is presented based on the close proximity in energy of ferromagnetic metallic phases, and also on percolative ideas valid at large disorder strength. The scenario is compared with previously discussed insulator-to-metal transitions in other contexts. It is argued that the effect unveiled here has unique properties that may define a new class of giant effects in complex oxides. This particularly severe effect of disorder must be present in other materials as well, in cases involving phases that arise as a compromise between very different tendencies, as it occurs with striped states in the cuprates.