Low-energy interband transitions inYBa2Cu3O7

Based on a numerically highly accurate local-density-approximation (LDA) calculation, we present a k⋅p analysis of the interband optical absorption in ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ in the infrared region (ħωE⊥c) polarization at about 320 ${\mathrm{cm}}^{\mathrm{-}1}$, a wide maximum for E∥c at about 420 ${\mathrm{cm}}^{\mathrm{-}1}$, and a structureless absorption (${\mathrm{\epsilon }}_2$≊const) for E∥c. The first feature is due to transitions between the apical-oxygen-derived bands, the second one to that between the z-even and z-odd ${\mathrm{CuO}}_2$ plane bands, and the third one to the transitions between the chain band and the z-odd plane band. The possibility of observing these features in experiment is discussed.