Superconductivity and the valence state of the Sr site in defectiveBi1.8Pb0.2Sr<

Abstract

The superconductivity and the average valence state of the Sr site are comparatively investigated for the La-doped Bi 1.8 Pb 0.2 Sr 2Ϫx La x CuO y (0.1рxр0.8) system and Sr-vacant Bi 1.8 Pb 0.2 Sr 2Ϫx CuO y (0рxр0.6) system. Electronic diffraction patterns and Raman spectra show that the incommensuration modulation ͑IM͒ is intensified in both systems, but the variation of the IM wavelengths in the La-doped system is much larger than in the Sr-vacant system. Resistivity measurements display that the superconductivity in the La-doped system appears from an overdoped region to underdoped region; however, it is strongly suppressed in the Sr-vacant system. Experimental analysis indicates that such differences between the two systems may arise from the fact that the La doping is pure electron filling, but the Sr vacancy in the Sr site brings about a shift of the apical oxygen of the CuO 6 octahedron and therefore the conductivity of the CuO 2 planes is directly influenced.