Electronic carrier density in doped Bi-based high-Tc superconductors

The superconducting carrier density n , was measured by means of μ + SR spectroscopy in the mixed phase of a series of polycrystalline Bi-based 2212 superconductors with different substitution of Bi 3+ by Pb 2+ . In exactly the same samples the effective valence per Cu-atom V ( Cu ) was determined by neutron powder diffraction. We observed that n , and V ( Cu ) are not proportional to each other in the heavily doped region. This behavior is interpreted as evidence that the carriers become more and more localized with increasing lead content.