Bilayer manganites: polarons in the midst of a metallic breakdown

TL;DR

This paper investigates the electronic phase of bilayer manganites, revealing that polaronic effects dominate in the CMR region, with metallic behavior emerging only in specific stacking configurations due to a breakdown of polaronic dominance.

Contribution

It provides new experimental evidence showing polaronic degrees of freedom dominate in La{2-2x}Sr{1+2x}Mn2O7 across the CMR region, highlighting the role of stacking number in metallic behavior.

Findings

Polaronic effects dominate in the CMR region.

Metallic behavior appears only in specific stacking configurations.

Incoherent charge carriers explain anomalous transport and spectroscopic data.

Abstract

The exact nature of the low temperature electronic phase of the manganite materials family, and hence the origin of their colossal magnetoresistant (CMR) effect, is still under heavy debate. By combining new photoemission and tunneling data, we show that in La{2-2x}Sr{1+2x}Mn2O7 the polaronic degrees of freedom win out across the CMR region of the phase diagram. This means that the generic ground state is that of a system in which strong electron-lattice interactions result in vanishing coherent quasi-particle spectral weight at the Fermi level for all locations in k-space. The incoherence of the charge carriers offers a unifying explanation for the anomalous charge-carrier dynamics seen in transport, optics and electron spectroscopic data. The stacking number N is the key factor for true metallic behavior, as an intergrowth-driven breakdown of the polaronic domination to give a metal possessing a traditional Fermi surface is seen in the bilayer system.