LATTICE DIELECTRIC FUNCTIONS OF ND2CUO4 AND LA2CUO4 BASED ON THE SHELL-MODEL

We present a theoretical study of the wave-vector and frequency depend ent lattice dielectric functions of Nd2CuO4 and La2CuO4. The calculati on is carried out using a formula we derived recently based on the she ll model description of the lattice dynamics. Except for damping const ants, this formula does not require adjustable parameters beyond those employed in the calculation of vibrational frequencies. For the case of q = 0, our calculated optical proper-ties are in good agreement wit h experimental measurements. For cases of q not-equal 0, our results i ndicate that both epsilon(xx)(q, omega) and epsilon(zz)(q, omega) are not very strongly anisotropic for frequencies higher than 20 meV and t he real part of the dielectric function is negative along the q(x) and q(z) directions for omega around 40 approximately 50 meV.