Pseudogap formation in 4d transition metal oxide BaRuO₃

Optical spectra of nine-layer BaRuO₃ show clear electrodynamic response changes due to a pseudogap formation. As temperature decreases, the pseudogap formation induces a gradual reduction of carrier density n and an abrupt, suppression of scattering rate 1/τ. It was found that the competition between n and 1/τ can induce a crossover from metallic to insulator-like regions. A clear development of the pseudogap as well as the coherent component in BaRuO₃ is evidently distinguished from other pseudogap phenomena observed in high T_c cuprates and heavy-electron systems.