LATTICE PROPERTIES OF YBA2CU3-XCOXO7 - PILOT CALCULATIONS USING EFFECTIVE-MEDIUM THEORY

We report preliminary total energy calculations for YBa2Cu3-xCoxO7 of the lattice parameters and of the Raman-active phonon modes, particula rly of the apex oxygen, within the so-called effective-medium theory. Experimentally it is found that substitution of Cu(l) by Co (or Al) in troduces a correlation between the apex oxygen mode and the critical t emperature, and that this correlation reflects the variation in distan ce between the O(4) and Cu(2), that decreases with doping, and between the Ba and Cu(2), that on the contrary increases with doping. The pic ture that emerges from this study is the following: as the Co (or Al) atoms partially substitute the Cu(l), there is a reduction of the hole carriers in the plane. This picture is supported by the present pilot calculation.