THEORETICAL CALCULATION OF THE WAVE-VECTOR AND FREQUENCY-DEPENDENT DIELECTRIC FUNCTION OF YBA2CU3O7

Real and imaginary parts of the dielectric function epsilon(omega-, q) of YBa2Cu3AO7 are calculated in the random-phase approximation, based on a parametrized tight-binding band structure. Only the contribution to the dieletric function from electronic interband transitions is considered. Prominent optical absorption peaks are found at 3.0, 2.3, and below 0.6 eV. For energy in the range 0.2-1.5 eV, we find that epsilon-1 is negative in a very large region inside the q(x), q(y) plane. Detailed studies on these low-energy absorption peaks reveal that they are mainly associated with the interband charge excitations involving orbitals within the Cu-O plane. For optical properties (q = 0), our results are compared with experimental data and previously reported theoretical calculations. There is a reasonably good agreement.