Insulator-Metal Phase Diagram of the Optimally Doped Manganites from the Disordered Holstein-Double Exchange Model

TL;DR

This paper maps the insulator-metal phase diagram of optimally doped manganites using a disordered Holstein-Double Exchange model, revealing how disorder influences polaron formation, magnetic transition temperatures, and transport properties.

Contribution

It introduces a new Monte Carlo method to analyze the phase diagram of the disordered Holstein-Double Exchange model, connecting theoretical predictions with experimental data on manganites.

Findings

Disorder significantly affects ferromagnetic T_c and metal-insulator transitions.

Polaron formation is enhanced by disorder, impacting transport regimes.

A comprehensive global phase diagram organizes diverse experimental observations.

Abstract

We study the Holstein-Double Exchange model in three dimensions in the presence of substitutional disorder. Using a new Monte Carlo technique we establish the phase diagram of the clean model and then focus on the effect of varying electron-phonon coupling and disorder at fixed electron density. We demonstrate how extrinsic disorder controls the interplay of lattice polaron effects and spin fluctuations and leads to widely varying regimes in transport. Our results on the disorder dependence of the ferromagnetic T_c and metal-insulator transitions bear direct comparison to data on the `optimally doped', x=0.3-0.4, manganites. We highlight disorder induced polaron formation as a key effect in these materials, organise a wide variety of data into a simple `global phase diagram', and make several experimental predictions.