METAL-INSULATOR-TRANSITION IN DOPED BISMUTHATE SUPERCONDUCTORS

We study the metal-insulator transition of BaBi1-xPbxO3 and Ba1-xKxBiO 3 within a Holstein molecular crystal model. The coherent potential ap proximation is used to calculate the density of states upon doping in both cases. It is found that in the Pb substituted compound the gap is filled up with impurity states while the pseudo-gap structure survive s even in the high doping regime. In the K-doped compound we consider the formation of polaron states in the vicinity of the band edges lead ing to a rapid decrease of the CDW gap upon doping. The relation to th e cuprate superconductors is discussed.