DEPRESSION OF SUPERCONDUCTIVITY AND THE PHONON-DRAG EFFECT IN GLASS-CERAMIC SUPERCONDUCTORS OBTAINED BY ANNEALING THE FE-DOPED (BI, PB)(4)SR3CA3CU4-MFEMOX GLASSY PRECURSOR

Electrical conductivities and Seebeck coefficients of Fe-doped Bi-2212 type superconductors have been reported in the temperature range 300- 10 K. These superconductors are obtained by annealing the Fe-doped amo rphous Bi3PbSr3Ca3Cu4-mFemOx (m = 0.0, 0.02, 0.04, 0.05, and 0.06) sys tem around 840 degrees C for 40 h in air. The superconducting transiti on temperature T-c decreases continuously with a gradual increase of F e content and finally diminishes for m greater than or equal to 0.06 s upporting the Abrikosov-Gor'kov-type pair-breaking mechanism. Unlike u ndoped glass ceramics, semiconducting behavior exhibited by all the Fe -doped glass ceramics in their normal states can be explained by Mott' s variable range hopping model. The Seebeck coefficients (S) of the gl ass-ceramic superconductors showed interesting nonlinear variation wit h temperature. Such nonlinear variation of thermopower is considered t o be associated with phonon-drag effect which is, however, not the cas e in undoped (Fe free) glass-ceramic superconductors. Formation of imp urity bonds such as Cu-O-Fe, Fe-O-Fe, etc., is considered to be more i mportant than the magnetic effect for the depression of T-c and nonlin ear thermal variation of Seebeck coefficient (S) in these superconduct ors. [S0163-1829(98)03041-0].