Localized character of the 'metallic' state in high Tc superconductors

The resistivity behaviour of a large series of polycrystalline samples at different defect (carrier) concentrations is discussed using the form p0 exp(T0/T)b in which p0 is a temperature-dependent parameter. This equation allows to examine the gradual and characteristic change of p(T) as a function of carrier concentration observed in single-crystals as well as in polycrystalline superconductors within an single mechanism. A cross-over between the b= 1/2 and the b= 1/4 regimes is found. The 'metallic' dependence of the normal resistivity state has a induced-disorder localized character.