Superconductivity and X-ray photoelectron spectroscopy studies of Bi2Sr2-xLaxCaCu2O8+δ

Abstract

The crystal structure (determined from X-ray diffraction measurements), the superconducting properties (from resistivity measurements) and electronic structure (from X-ray photoelectron spectroscopy investigation) of Bi2Sr2-xLaxCaCu2O8 compounds (substituted on the strontium site) have been studied for different lanthanum concentrations. A systematic increase in the lattice parameter a and a decrease in the c-axis with increasing x was observed. The ρ-T characteristics for different x showed that Tc increases up to 91 K (x=0.2) then falls for higher doping, indicating that the substitution induces a decrease in the hole density. The superconductor-insulator transition occurred at a higher doping level (x=0.7) than in the case of a substitution for calcium. This behaviour has been correlated to the change in the electronic structure. This was revealed by a lowering of the Fermi level and a shift towards lower energy of the core level X-ray photoelectron spectra Bi 4f. The different effect of the strontium substituted site in comparison with that of the calcium site has been tentatively explained by their respective crystallographic situations giving rise to different charge transfer and coupling between Bi-O and Cu-O2 planes.