Substitution effect on superconductivity in Fe-doped Bi-system superconductors

The electronic structures of FeO 4 Ca 8 and FeO(1) 4 O(2)Ca 4 Sr 4 clusters in the Fe-doped Bi-systemhigh- T c phase are calculated using the SCF-Xα-SW method. From the calculated result, we find that the Fe atom does not contribute to the density of states near the Fermi energy in both clusters. Fe substitution for Cu sites of Bi-2233 phase is highly deleterious to superconducting behavior. The energy gap of FeO(1) 4 O(2)Ca 4 Sr 4 on the Fermi energy is about seven times larger than that of FeO 4 Ca 8 . Moreover, for the FeO(1) 4 O(2)Ca 4 Sr 4 cluster, the peak of the density of states (DOS) in the high energy range shifts to the low energy range and forms new DOS peaks, in contrast to the density of states in the FeO 4 Ca 8 cluster. The bonding of the hybridized orbital in FeO(1) 4 O(2)Ca 4 Sr 4 is stronger than in FeO 4 Ca 8 , namely the FeO(1) 4 O(2)Ca 4 Sr 4 is even stabler than the FeO 4 Ca 8 cluster. Such calculated result provides satisfactory agreement with Fe-doped experiments. It is observed that T c decreases with increasing iron content and Fe dopant gives rise to enhancement of the 2212 phase at expense of the 2223 in Bi-system superconductors. The calculated results can help us to understand the nature of Fe-doping for Cu on quantum mechanics background. Furthermore the theoretical value of the electric field gradient given by the SCF-Xα-SW method is agreement with the value deduced from quadrupole splittings by Mossbauer spectroscopy. This calculation can provide theoretical support for assignment of Fe species to Cu sites in high- T c superconductors.