Scaling of the characteristic energy in superconducting YBa ₂Cu ₃O _7-δ and HgBa ₂CaCu ₂O _6+δ

We have performed temperature-dependent reflectivity measurements on HgBa ₂CaCu _26+δ thin films (T _c = 125 K). A gap-like feature is found in the superconducting to normal state reflectivity ratio R _s/R _n. This feature occurs at ∼ 700 cm ^-1 (≡ 2Δ), a value higher than that of YBa ₂Cu ₃O _7-δ (2Δ = 500 cm ^-1, T _c = 90 K), and scaling holds between 2Δ and T _c for these two compounds. The feature disappears suddenly as the temperature is raised above T _c. These observations strongly suggest that the gap-like feature is associated with the mechanism underlying the superconductivity in high-T _c copper oxides.