Temperature-dependent Raman spectroscopy in BaRuO₃ systems

We investigated the temperature dependence of the Raman spectra of a nine-layer BaRuO₃ single crystal and a four-layer BaRuO₃ 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 RuO₆. 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 BaRuO³. And, the temperature dependence of the Raman continua between 250 and 600 m-^-1is 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 BaRuO₃ systems.