Comment on ''Phase Diagram of La$_{2-x}$Sr$_x$CuO$_4$ Probed in the Infrared: Imprints of Charge Stripe Excitations''

TL;DR

This paper critiques recent infrared reflectivity studies of La$_{2-x}$Sr$_x$CuO$_4$, revealing that observed spectral peaks attributed to charge stripes and polarons are actually artifacts caused by measurement leakage.

Contribution

It clarifies that previously reported infrared peaks are due to experimental artifacts rather than intrinsic charge stripe or polaronic excitations.

Findings

Reported peaks are caused by leakage of c-axis reflectivity.

Spectral features previously linked to charge stripes are artifacts.

Reassessment of infrared data alters understanding of charge excitations.

Abstract

Recently Lucarelli {\it et al.} have reported\cite{lucarelli} temperature-dependence of the in-plane optical reflectivity of La$_{2-x}$Sr$_x$CuO$_4$ over a wide doping range, focusing on the infrared peaks at 30 cm$^{-1}$ (for $x$=0.12), 250 cm$^{-1}$ and 510 cm$^{-1}$. They interpreted the first peak (30 cm$^{-1}$) as a signature of charge stripe ordering, while the latter two (250 cm$^{-1}$ and 510 cm$^{-1}$) are attributed to the polaronic charge excitations. However, careful readers would notice that the reported spectra are largely different from those so far measured on the same system. As we illustrate below, all these peaks are caused by an uncontrolled leakage of the c-axis reflectivity into the measured spectra.