<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>a</mml:mi><mml:mo>−</mml:mo><mml:mi>b</mml:mi></mml:math>plane optical conductivity in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">YBa</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow><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:mi>−</mml:mi><mml:mi>δ</mml:mi></mml:mrow></mml:msub></mml:mrow></mml:math>above and below<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mrow><mml:mi>T</mml:mi></mml:mrow><mml:mrow><mml:mi>*</mml:mi></mml:mrow></mml:msup></mml:mrow></mml:math>

Analysis of the $a\ensuremath{-}b$ plane optical conductivity ${\ensuremath{\sigma}}_{\mathrm{ab}}$ for both twinned and untwinned ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ as a function of temperature and doping shows that below a well-defined temperature ${T}^{*},$ a dip in the spectrum systematically appears separating the infrared charge excitation spectrum into two components with distinct energy scales. The change from monotonic behavior in ${\ensuremath{\sigma}}_{\mathrm{ab}}$ is found to be concurrent with the onset of phonon anomalies in Raman and infrared spectra below ${T}^{*}$. The optical data are suggested to be evidence for the appearance of an inhomogeneous distribution of carriers rather than the opening of a simple gap for charge excitations below ${T}^{*}$, an interpretation which is consistent with recent angle-resolved photoemission and electronic Raman spectra. We find that the behavior below ${T}^{*}$ and the absence of any anomalies at ${T}_{c}$ can be interpreted assuming a Bose-Einstein condensation of preformed pairs.

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