Signatures of electronic polarons in La$_{1-x}$Sr$_{1+x}$MnO$_4$ observed by electron energy-loss spectroscopy

TL;DR

This study uses electron energy-loss spectroscopy to identify electronic polarons in La$_{1-x}$Sr$_{1+x}$MnO$_4$, revealing their signatures, dispersion, and coupling with magnetic properties, advancing understanding of charge dynamics in doped manganites.

Contribution

It provides direct spectroscopic evidence of electronic polarons and their interactions in La$_{1-x}$Sr$_{1+x}$MnO$_4$, highlighting the coupling between charge transfer and magnetic correlations.

Findings

Signature of electronic polarons at 1.65 eV energy loss.

Doping-induced excitation propagates within the ab-plane.

Polaron excitation shifts with magnetic correlations.

Abstract

The dielectric properties of La$_{1-x}$Sr$_{1+x}$MnO$_4$ single crystals with x = 0, 0.125, 0.25, and 0.5 were studied by means of electron energy-loss spectroscopy as a function of temperature and momentum transfer. A clear signature of the doped holes is observed around 1.65 eV energy loss, where spectral weight emerges with increasing x. For all $x \neq 0$, this doping-induced excitation can propagate within the ab-plane, as revealed by a clear upward dispersion of the corresponding loss peak with increasing momentum transfer. The hole-induced excitation also shifts to higher energies with the onset of magnetic correlations for x = 0.5, implying a strong coupling of charge and spin dynamics. We conclude that (i) the loss feature at 1.65 eV is a signature of electronic polarons, which are created around doped holes, and that (ii) this low-energy excitation involves the charge transfer between manganese and oxygen. The finite dispersion of these excitations further indicates significant polaron-polaron interactions.