Temperature-dependent Raman spectroscopy in BaRuO$_3$ systems

TL;DR

This study explores how Raman spectra of BaRuO$_3$ systems change with temperature, revealing phonon behavior, structural instability, and their relation to pseudogap formation in these layered compounds.

Contribution

It provides detailed temperature-dependent Raman analysis of BaRuO$_3$ single crystal and film, linking phonon shifts and mode splitting to structural instability and pseudogap phenomena.

Findings

$A_{1g}$ modes harden with decreasing temperature

$E_g$ modes soften or remain unchanged

Splitting of an $E_{2g}$ mode at low temperatures

Abstract

We investigated the temperature-dependence of the Raman spectra of a nine-layer BaRuO$_3$ single crystal and a four-layer BaRuO$_3$ epitaxial film, which show pseudogap formations in their metallic states. From the polarized and depolarized spectra, the observed phonon modes are assigned properly according to the predictions of group theory analysis. In both compounds, with decreasing temperature, while $A_{1g}$ modes show a strong hardening, $E_g$ (or $E_{2g}$) modes experience a softening or no significant shift. Their different temperature-dependent behaviors could be related to a direct Ru metal-bonding through the face-sharing of RuO$_6$. It is also observed that another $E_{2g}$ mode of the oxygen participating in the face-sharing becomes split at low temperatures in the four layer BaRuO$_3 $. And, the temperature-dependence of the Raman continua between 250 $\sim $ 600 cm$^{-1}$ is strongly correlated to the square of the plasma frequency. Our observations imply that there should be a structural instability in the face-shared structure, which could be closely related to the pseudogap formation of BaRuO$_3$ systems.