Existence of orbital polarons in ferromagnetic insulating La$_{1-x}$Sr$_x$MnO$_{3}$ (0.11$<x<$0.14) evidenced by giant phonon softening

TL;DR

This study reveals giant phonon softening in ferromagnetic insulating La$_{1-x}$Sr$_x$MnO$_3$ due to orbital polarons, providing evidence of their existence through inelastic light scattering measurements.

Contribution

It provides experimental evidence for orbital polarons in ferromagnetic insulating manganites via phonon softening analysis.

Findings

Giant phonon softening observed in FMI samples below Curie temperature.

Softening is suppressed with increased Pr-doping.

Coupling between breathing mode and orbital polarons is identified.

Abstract

We present an inelastic light scattering study of single crystalline (La$_{1-y}$Pr$_y$)$_{1-x}$Sr$_{x}$MnO$_3$ ($0\leq x\leq0.14$,$y=0$ and $x=1/8$,$0\leq y\leq0.5$). A giant softening up to 20 - 30 cm$^{-1}$ of the Mn-O breathing mode has been observed only for the ferromagnetic insulating (FMI) samples ($0.11\leq x \leq 0.14$) upon cooling below the Curie temperature. With increasing Pr-doping the giant softening is gradually suppressed. This is attributed to a coupling of the breathing mode to orbital polarons which are present in the FMI phase.