Striping of orbital-order with charge-disorder in optimally doped manganites

TL;DR

This paper reveals how orbital-order striping with charge disorder in optimally doped manganites leads to the melting of long-range orbital order, explaining the transition to a conducting state associated with colossal magnetoresistance.

Contribution

It demonstrates that superposition of lattice modes causes orbital-order striping and charge disorder, providing a microscopic mechanism for the loss of orbital order at optimal doping.

Findings

Superposition of lattice modes causes orbital-order striping.

Charge disorder emerges at the critical doping level.

Orbital-order melting facilitates transition to a conducting state.

Abstract

The phase diagrams of LaMnO$_3$ perovskites have been intensely studied due to the colossal magnetoresistance (CMR) exhibited by compositions around the $\frac{3}{8}^{th}$ doping level. However, phase segregation between ferromagnetic (FM) metallic and antiferromagnetic (AFM) insulating states, which itself is believed to be responsible for the colossal change in resistance under applied magnetic field, has prevented an atomistic-level understanding of the orbital ordered (OO) state at this doping level. Here, through the detailed crystallographic analysis of the phase diagram of a prototype system (AMn$_3^{A'}$Mn$_4^B$O$_{12}$), we show that the superposition of two distinct lattice modes gives rise to a striping of OO Jahn-Teller active Mn$^{3+}$ and charge disordered (CD) Mn$^{3.5+}$ layers in a 1:3 ratio. This superposition only gives a cancellation of the Jahn-Teller-like displacements at the critical doping level. This striping of CD Mn$^{3.5+}$ with Mn$^{3+}$ provides a natural mechanism though which long range OO can melt, giving way to a conducting state.