$a-b$ plane optical conductivity in ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ above and below $T^{*}$

Analysis of the $a-b$ plane optical conductivity ${\sigma }_{\mathrm{ab}}$ for both twinned and untwinned ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ 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 ${\sigma }_{\mathrm{ab}}$ 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.