Photoemission study of R BaCo2 O6-δ (R=Tb, Nd; δ=0.5, 1.0)

We have studied the electronic structure of the oxygen-deficient type perovskite TbBaCo2 O5.5, NdBaCo2 O5.5, and NdBaCo2 O5 using x-ray photoemission spectroscopy. The valence-band photoemission spectrum of TbBaCo2 O5.5 shows the gap opening at the Fermi level and the spectral-weight transfer from the eg band to the t2g band, across the metal-insulator transition at 340 K. The valence-band spectra of R BaCo2 O6-δ commonly have broad Co 3d bands, suggesting that the Co 3d electrons are distributed to the t2g and eg bands even in the insulating phase. The present results indicate that the metallic versus insulating behaviors of R BaCo2 O5.5 are related to the energy separation between the t2g and eg bands.