Anomaly of oxygen bond-bending mode at 320 cm^-1 and additional absorption peak in the c-axis infrared conductivity of underdoped YBa₂Cu₃O_7-δ single crystals revisited with ellipsometric measurements

We have performed ellipsometric measurements of the far-infrared c-axis dielectric response of underdoped YBa₂Cu₃O_7-δ single crystals. Here we report a detailed analysis of the temperature-dependent renormalization of the oxygen bending phonon mode at 320 cm^-1 and the formation of the additional absorption peak around 400–500 cm^-1. For a strongly underdoped YBa₂Cu₃O_6.5 crystal with T_c=52 K we find that, in agreement with previous reports based on conventional reflection measurements, the gradual onset of both features occurs well above T_c at T^*∼150 K. Contrary to some of these reports, however, our data establish that the phonon anomaly and the formation of the additional peak exhibit very pronounced and steep changes right at T_c. For a less underdoped YBa₂Cu₃O_6.75 crystal with T_c=80 K, the onset temperature of the phonon anomaly almost coincides with T_c. Also in contrast to some previous reports, we find for both crystals that a sizeable fraction of the spectral weight of the additional absorption peak cannot be accounted for by the spectral-weight loss of the phonon modes but instead arises from a redistribution of the electronic continuum. Our ellipsometric data are consistent with a model where the bilayer cuprate compounds are treated as a superlattice of intrabilayer and interbilayer Josephson junctions.