Chain-site versus plane-site Cu substitution inYBa2Cu3−xMxO7(M=Co,Ni): Hall and thermopower studies

Substitution of Cu by Ni at Cu(2) (plane) sites in Y${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ has a weak effect on the Hall constant, but turns the thermopower negative. In contrast, substitution at Cu(1) (chain) sites by Co decreases the Hall number exponentially. The suppression of $T_c$ is due to different mechanisms in the two cases. We discuss how doping affects the carrier density, and the evidence in favor of a filling factor of ½ in the undoped system.