0.7 eV excitation in YBa₂Cu₃O₇-δ: evidence from epitaxial thin film and powder samples

We have measured the optical absorption spectrum of YBa2Cu3O7-delta crystalline thin films and powders in the range 2000 to 10000 cm-1. A broad absorption band near 5500 cm-1 (0.7 eV) is observed, which has the same absorption profile in both cases. Our work on thin-film samples shows that the excitation causing this absorption is polarized in the ab plane. Further work on doped powder samples provides evidence that the excitation is localized in the CuO2 planes of the structure, which are known to be the principal carriers of the superconductivity. Studies of the temperature dependence of the absorption have revealed that the peak intensity undergoes a transition at the superconducting transition temperature, providing firm evidence that the excitation is coupled to the superconducting order parameter