EVIDENCE FOR CHARGE-TRANSFER IN BI-BASED SUPERCONDUCTORS STUDIED BY POSITRON-ANNIHILATION

We have measured Doppler-broadening annihilation radiation (DBAR) spec tra and positron lifetimes in normal and superconducting states for th ree kinds of Bi-based superconductors: Bi2212, Pb-doped Bi2223, Pb- an d F-doped Bi2223. The difference spectra after deconvolution between t wo states show a sharpening effect with increasing temperature; the F- doped sample has the greatest amplitude in difference spectra but near ly the same positron lifetimes as the Pb-doped sample. The results are interpreted in terms of charge transfer between the Cu-O and Bi-O pla nes. The role of oxygen defects in charge transfer is discussed.