LOW-ENERGY INTERBAND-TRANSITIONS IN YBA2CU3O7

Based on a numerically highly accurate local-density-approximation (LDA) calculation, we present a k.p analysis of the interband optical absorption in YBa2Cu3O7 in the infrared region (HBARomega < 0.1 eV). It is shown that the LDA band structure gives rise to three infrared interband transitions: A sharp peak for the in-plane (E perpendicular-to c) polarization at about 320 cm-1, a wide maximum for E parallel-to c at about 420 cm-1, and a structureless absorption (epsilon2 almost-equal-to const) for E parallel-to 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 CuO2 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.