Role of interlayer coupling in alkaline-substituted (Bi, Pb)-2223 superconductors

In this work, we investigated a correlation between superconductivity and interlayer coupling of two different alkaline (Na and K)-substituted Bi1.6Pb0.4Sr2Ca2Cu3O10+delta (BSCCO) polycrystalline samples. The excess conductivity analysis by the Aslamazov-Larkin (AL) and Lawrence-Doniach (LD) theories showed that Na substitution at the Ca site induced a gradual broadening of 3D fluctuation region with increasing interlayer coupling strength, which explains a systematic increase of T-c and a decrease of normal state resistivity. On the other hand, exactly the opposite results were observed in the K-substituted samples in place of Sr. Extended x-ray absorption fine structure (EXAFS) studies revealed that substitution of Na and K generated completely different effects on the local structure around Cu atoms. It is noticeable that the Cu-O bond distance was found to decrease monotonically with the varying amounts of Na, which indicates that the CuO2 layer is stabilized. On the while, the opposite was observed to occur with the varying amounts of K. Unlike the Cu-Ca bond which was the least affected by the substitution, the Cu-Sr bond distance increased drastically with K substitution. All these findings indicate that Na substitution at the Ca site enhances superconductivity with no loss of interlayer interaction, while K substitution at the Sr site weakens superconductivity due to the diminished interlayer interaction. (C) 2019 Elsevier B.V. All rights reserved.