Optical characterization of the charge density wave phase of 9R BaRuO₃ thin film

We report on optical spectroscopic study of a nine-layered rhombohedral (9R) BaRuO₃ (BRO) thin film. The electronic response of the 9R BRO thin film displays the representative behaviors of the charge density wave (CDW) transition. We observe the formation of a partial optical gap at T* = ∼ 140 K, at which the dc resistivity showed a metal-insulator transition. As the temperature decreases below 140 K, the spectral weight (SW) and the scattering rate of the Drude-like peak are suppressed. The SW removed from the Drude-like peak is transferred into the energy region between 55 and 150 meV to form a gap-like structure. The magnitude of the CDW gap is found to be about 85 meV, corresponding to 2Δ/k_BT* ≈ 7.56. The large 2Δ/k_BT* value suggests that the many-body effects can be involved in this system. Indeed, we find a signature of the electron-phonon coupling: a pronounced asymmetry of a phonon mode at about 67 meV. The degree of asymmetry is enhanced as the temperature decreases with a clear anomaly at T*. The temperature evolution of the phonon mode suggests that the coupling with the electronic excitations whose SW increases due to the partial gap formation is responsible for the anomaly of the phonon mode. Our results imply that electron-phonon coupling is an important ingredient for the CDW order of 9R BRO.