HIGH-RESOLUTION STM STS STUDIES OF BI-O REDOX LAYERS AND SUPERCONDUCTIVITY IN PURE AND SUBSTITUTED BI-2122 HIGH-TC CUPRATE

Effects arising from the intercalation of excess oxygen in Bi-O redox layers of the Bi-2122 compound have been studied with a STM/STS technique which has been successfully used for obtaining atomically resolved images. The excess oxygen is incorporated by substituting Dy3+ at the Ca2+-site as well as by oxygenating the pure sample. It has been possible to detect and directly observe the excess oxygen atoms in the Bi-O layer which is in agreement with the information inferred from neutron diffraction and HRTEM studies of other workers. The STS studies show that there is a gradual increase in its room temperature energy gap and a decrease in the LDOS, indicating the Bi-O layer to become insulating with intercalation of excess oxygen. Depletion of excess oxygen through nitrogenation or Pb substitution at the Bi-site produces the reverse effect. The importance of improving the metallicity of the redox layer is elaborated and it is demonstrated that the T(c) of the bulk Bi-2122 phase can be significantly sharpened and raised to 96 K by depleting the excess oxygen from the layer. The redox reaction and the influence of the resistive state of the Bi-O layers in relation to the observed superconductivity results are discussed.