Abstract
It has been reported that the superconductivity of Bi2Sr2Ca2Cu3O10+δ (Bi-2223) can be further enhanced by cation doping with alkaline material such as Na-doping at Ca sites. We investigated the mechanism of the enhancement of superconductivity in Bi1.6Pb0.4Sr2Ca2-xNaxCu3O10+δ (x = 0.00 to 0.06) in terms of valence state and local atomic displacement by way of Cu K-edge X-ray absorption fine structure (XAFS) spectroscopy. The usage of the extended XAFS regularization method enabled us to obtain the pair distribution function and the effective interatomic potential of the in-plane Cu–O pairs. The analysis of XAFS spectra revealed that distortion in the CuO2 plane is relieved as Na contents increase, thereby providing a more stable structure. Furthermore, upon Na-doping, the in-plane Cu–O bond becomes contracted, accompanied by an increase of the average valence states. As a consequence, the increase in the Cu valence by Na-doping seems to induce an adjustment of the CuO2 plane in a direction to drive the superconductivity to enhance.
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