Charge Dynamics of the High‐Temperature Superconductor YBa2Cu3Oy

We have studied the charge dynamics of the high‐temperature superconductor YBa2Cu3Oy (YBCO) by measuring optical reflectivity at various temperatures from the insulating sample (y = 6.3) to the optimally‐doped sample (y = 6.91). We have found that the spectral weight of the optical conductivity increases in the narrow low‐energy region (ℏω < 0.06 eV) with decreasing temperature. This behavior is attributed to an an evolution of the coherent quasiparticle band with temperature.