DIELECTRIC TENSOR OF YBA2CU4O8 - EXPERIMENT AND THEORY

In this work we present measurements of the anisotropic optical respon se of YBa2Cu4O8. By using ellipsometry and reflectance measurements, w e obtained all three components of the dielectric tensor in the energy range between 0.05 and 5.5 eV. We report the existence of a plasma ed ge for light polarized perpendicular to the CuO2 planes. For the in-pl ane optical response, Drude-Lorentz fits to our reflectance spectra sh ow a behavior that differs qualitatively from that reported for YBa2Cu 3O7, namely that the scattering frequency for the carriers in the chai n is smaller than for the plane carriers. We assign this to the stabil ity of the chains against oxygen loss and disorder. We have isolated t he contribution of interband transitions to the dielectric function an d compare it to results from an ab initio calculation. For the computa tion of the one-electron states we used the linear-muffin-tin-orbital method in the atomic-sphere approximation. We find good agreement betw een the calculated and the measured spectra, hinting at the validity o f the underlying one-electron local-density approximation. We give a m icroscopic explanation of the observed structure in the optical respon se and point out the main differences between the band structures of Y Ba2Cu4O8 and YBa2Cu3O7 as they manifest themselves in the optical prop erties.