Effects of post-annealing on the infrared active phonon modes of low anisotropy ( γ=5–11) Cu 1− xTl xBa 2Ca 2Cu 3O 10− δ superconductor thin films

Abstract

The effects of post-annealing conditions on the apical oxygen phonon modes have been investigated and their correlations with the critical temperature of the Cu 1− x Tl x Ba 2Ca 2Cu 3O 10− δ (Cu 1− x Tl x -1223) superconductor thin films have been developed. The samples were annealed in different ambient environments, such as air, N 2, O 2 and vacuum and their FTIR absorption spectrum has been taken. Three absorption peaks observed between 455–440, 665–648 and 1240–1246 cm −1 are assigned to apical oxygen vibrations, the chain axis O(3) vibrations and to C–O single-bond, respectively. The frequency of the mode at 655 cm −1 is sensitive to the concentration of O(3) in the charge reservoir layer, which in turn controls increase or decrease of the bond length of apical oxygen. The softening of the apical oxygen mode is possibly linked with the reduction of thallium, which in turn is associated with the amount of oxygen [O(3)] in the charge reservoir layer. In as-prepared material and annealed under oxygen, the state of thallium is not reduced from Tl +3 to Tl +1 and sufficient number of carriers are not supplied to CuO 2 planes, the material remains in the region of carrier under-doping. The softening of apical oxygen mode positioned at 455 cm −1 is associated with the formation of low T c phases. The C–O related IR mode is softened with the post-annealing in air at 780 °C and annealing in N 2 ambient at 400 °C. The critical transition temperature is found to decrease with the post-annealing at 400 °C in N 2, which is possibly due to the under-doped CuO 2 planes of the material. However, the post-annealing at 780 and 650 °C increases T c (mid-point) to 107 and 111 K respectively.