Evidence for a superconducting origin of prominent features of the in-plane infrared response of underdoped cuprates and implications of their persistence above T_(c)

We report on results of our analysis of published experimental data of the in-plane infrared response of two representative underdoped high-$T_{\mathrm{c}}$ cuprate superconductors, focusing on a characteristic gap feature in the spectra of the real part of the conductivity and the corresponding structures of the memory function, that develop below a temperature $T^{\mathrm{ons}}$ considerably higher than $T_{\mathrm{c}}$. Several arguments based on comparisons of the data with results of our calculations are provided indicating that the features are due to superconductivity and that $T^{\mathrm{ons}}$ marks the onset of a precursor superconducting phase.