<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Ba</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow><mml:mi mathvariant="normal">Y</mml:mi><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">Cu</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub></mml:mrow><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">O</mml:mi></mml:mrow><mml:mrow><mml:mn>7</mml:mn><mml:mo>−</mml:mo><mml:mi>δ</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math>: Electrodynamics of Crystals with High Reflectivity

We present results for the reflectivity, $R$ of two bulk crystals of ${\mathrm{Ba}}_{2}\mathrm{Y}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ which, below the superconducting transition at ${T}_{c}$, show behavior consistent with perfect reflectivity up to an energy of $(3\ensuremath{-}4){k}_{\mathrm{B}}{T}_{c}$. We interpret the reproducibility and high $R$ values (especially $R\ensuremath{\sim}1$ 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.

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