SUBSTITUTION EFFECT ON SUPERCONDUCTIVITY IN FE-DOPED BI-SYSTEM SUPERCONDUCTORS

The electronic structures of FeO4Ca8 and FeO(1)4(O)(2)Ca4Sr4 clusters in the Fe-doped Bi-system high-T-c phase are calculated using the SCF- X alpha-SW method. From the calculated result, we find that the Fe ato m does not contribute to the density of states near the Fermi energy i n both clusters. Fe substitution for Cu sites of Bi-2223 phase is high ly deleterious to superconducting behavior. The energy gap of FeO(1)(4 )O(2)Ca4Sr4 on the Fermi energy is about seven times larger than that of FeO4Ca8. Moreover, for the FeO(1)(4)O(2)Ca4Sr4, cluster, the peak o f the density of states (DOS) in the high energy range shifts to the l ow energy range and forms new DOS peaks, in contrast to the density of states in the FeO4Ca8 cluster. The bonding of the hybridized orbital in FeO(1)(4)O(2)Ca4Sr4 is stronger than in FeO4Ca8 namely the FeO(1)(4 )O(2)Ca4Sr4 is even stabler than the FeO4Ca8 cluster. Such calculated result provides satisfactory agreement with Fe-doped experiments. It i s observed that T-c decreases with increasing iron content and Fe dopa nt gives rise to enhancement of the 2212 phase at expense of the 2223 phase in Bi-system superconductors. The calculated results can help us to understand the nature of Fe-doping for Cu on quantum mechanics bac kground. Furthermore the theoretical value of the electric field gradi ent given by the SCF-X alpha-SW method is in agreement with the value deduced from quadrupole splittings by Mossbauer spectroscopy. This cal culation can provide theoretical support for assignment of Fe species to Cu sites in high-T-c superconductors.