Superconductivity and charge transfer compensation by direct doping in the La 1+ xCa yBa 2− x− yCu 3O 7+δ system

Abstract

Substitution of La and Ca on the Y and Ba sites in the 1-2-3 structure permits the investigation of charge transfer in the solid solution La 1+ x Ca y Ba 2− x− y Cu 3O 7+δ for doping over a wide compositional range. The superconducting, structural and optical properties of this system are reported. Single-phase samples have been found for 0 < x < 0.4, depending on the Ca content and metallicity and superconductivity are preserved throughout the single-phase region. Changes in the charge balance due to La +3 substituting on the Ba 2+-site are compensated by increased oxygen content and the superconducting properties are maintained over a wide range of x and y due to the presence of Ca. From infrared and Raman spectroscopy, we find that the apex O(4) frequency does not exhibit significant shifts with composition. This is consistent with the hypothesis that the apex O(4) vibration is related to the electronic charge state of the CuO planes in the 1-2-3 structure.