Doping-dependent correlations between electronic transitions and superconductivity in La/sub 2-x/Sr/sub x/CuO/sub 4-//sub δ/

The high-temperature superconducting oxides are sensitively dependent on small percent differences in doping for their superconducting character. We have studied the optical properties of samples in which the doping of Sr in La/sub 2-//sub x/Sr/sub x/CuO/sub 4-//sub δ/, is varied. The techniques of reflectance and ellipsometric spectroscopy were used to cover the energy range from 0.3 eV to 6 eV. These revealed correlations between the electronic properties of the material and its ability to superconduct as quantified by the critical temperature and the Meissner effect. In particular, the strength of an optical absorption structure at ∼0.5 eV is found to vary with Sr content in parallel with the superconductivity of the material. Significantly, the energetic position of the structure is fixed for the broad range of doping studied, indicating that it is not simply a plasmon excitation of the carriers introduced by doping. These results suggest a common root between the electronic structure associated with the optical absorption and the mechanism for superconductivity