Abstract
Reflectivity of polycrystalline YBa2Cu3O7-δ has been measured in the energy ranges from 2.5 to 0.05 eV and from 27 to 3.7 meV at room temperature. A Drude edge due to the free electron has been observed at 1.4 eV and the carrier concentration is estimated as 3.2×1021 cm-3. The observed low reflectivity suggests the two dimensional character of the free electron. Below Tc a structure has been observed around 17.7 meV (143 cm-1), from which the energy gap 2Δ is tentatively estimated as 15.5 meV (125 cm-1) at 15 K. Alternatively, the structure may be due to a transverse collective excitation.