Backfolded acoustic phonons in metal-oxide superlattices

TL;DR

This paper reports the detection of backfolded acoustic phonons in Raman spectra of high-temperature superconductor and manganite superlattices, revealing their potential for characterizing superlattice properties and inhomogeneities.

Contribution

It demonstrates the observation of backfolded acoustic phonons in metal-oxide superlattices and their application in measuring bilayer thickness and film inhomogeneity.

Findings

Backfolded acoustic phonons observed in Raman spectra.

Modes can determine superlattice bilayer thickness.

Raman microscopy maps film thickness inhomogeneity.

Abstract

We report the observation of low-frequency modes in the Raman spectra of thin-film superlattices of the high-temperature superconductor YBa$ _{2} $Cu$ _{3} $O$ _{7-\delta} $ and various manganite perovskites. Our study shows that these modes are caused by the backfolding of acoustic phonons due to the additional periodicity introduced by the superlattice. Such modes were previously only observed for ultra-pure semiconductor superlattices. They can be used to determine the bilayer thickness of the superlattice and its speed of sound. Moreover, we use the spatial resolution of Raman microscopy to map the film thickness inhomogeneity across a sample, making these modes a useful tool to characterize thin-film superlattices.