Theory of high energy optical conductivity and the role of oxygens in manganites

TL;DR

This paper models the optical conductivity in manganites, highlighting oxygen's role in electron hopping and explaining spectral weight transfer during magnetic phase transitions using Dynamical Mean Field Theory.

Contribution

It introduces a model that explains spectral weight transfer in manganites by emphasizing oxygen-mediated hopping and its dependence on magnetization, aligning well with experimental data.

Findings

Oxygen mediates electron hopping between manganese ions.

Spectral weight transfer is linked to ferromagnetic ordering.

Model shows good quantitative agreement with experiments.

Abstract

Recent experimental study reveals the optical conductivity of La$_{1-x}$Ca$_x$MnO$_3$ over a wide range of energy and the occurrence of spectral weight transfer as the system transforms from paramagnetic insulating to ferromagnetic metallic phase [Rusydi {\it et al.}, Phys. Rev. B {\bf 78}, 125110 (2008)]. We propose a model and calculation within the Dynamical Mean Field Theory to explain this phenomenon. We find the role of oxygens in mediating the hopping of electrons between manganeses as the key that determines the structures of the optical conductivity. In addition, by parametrizing the hopping integrals through magnetization, our result suggests a possible scenario that explains the occurrence of spectral weight transfer, in which the ferromagnatic ordering increases the rate of electron transfer from O$_{2p}$ orbitals to upper Mn$_{e_g}$ orbitals while simultaneously decreasing the rate of electron transfer from O$_{2p}$ orbitals to lower Mn$_{e_g}$orbitals, as temperature is varied across the ferromagnetic transition. With this scenario, our optical conductivity calculation shows very good quantitative agreement with the experimental data.