Ba2YCu3O7−δ: Electrodynamics of Crystals with High Reflectivity

We present results for the reflectivity, $R$ of two bulk crystals of ${\mathrm{Ba}}_2\mathrm{Y}{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ which, below the superconducting transition at $T_c$, show behavior consistent with perfect reflectivity up to an energy of $\mathrm{}(3-4)\mathrm{}{k}_{\mathrm{B}}{T}_c$. We interpret the reproducibility and high $R$ values (especially $R\sim 1$ below $T_c$) as indicative of intrinsic behavior in the highly conducting crystal planes probed by the light. Results above $T_c$ show that the charge carriers have an enhanced effective mass and low scattering rate at low frequency and temperature, indicating a strong interaction with some excitation.