Electronic structure and optical properties of Y₂BaCuO₅ with antiferromagnetic spin arrangements

抄録

First-principles energy band calculations are performed for Y₂BaCuO₅ (YBC211) with ferromagnetic and three types of antiferromagnetic (AFM) spin orderings, namely, A, C, and G. The most stable phase is found to be G AFM ordering (AFM_G_YBC211). The calculated magnetic moment at the Cu sites is almost identical to the experimental one reported in the literature. However, the energy bandgap is estimated to be only 0.55 eV. The valence band (VB) maximum locates on the S-Γ line and the conduction band (CB) minimum locates at the Γ point, suggesting that AFM_G_YBC211 is an indirect bandgap oxide. From the density-of-states analysis, the VB of AFM_G_YBC211 comprises mainly the Cu 3d and O 2p states. The lower CB comprises the Cu 3d states, whereas the upper CB is attributed mainly to the Y 4d, Ba 5d, and O 2p states. Furthermore, in the dielectric function calculation of AFM_G_YBC211, the ε₂(ω) curves exhibit very strong peaks for the yy and zz components at 0.87 eV, whereas the corresponding peak for the xx component is weak. This difference is attributed to the anisotropy of the momentum matrix elements between the VB and the lower CB.