Anomaly of oxygen bond-bending mode at $320\hspace{0.5em}{\mathrm{cm}}^{-1}$ and additional absorption peak in the c-axis infrared conductivity of underdoped ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ single crystals revisited with ellipsometric measurements

We have performed ellipsometric measurements of the far-infrared c-axis dielectric response of underdoped ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ single crystals. Here we report a detailed analysis of the temperature-dependent renormalization of the oxygen bending phonon mode at $320\hspace{0.5em}{\mathrm{cm}}^{-1}$ and the formation of the additional absorption peak around $400–500\hspace{0.5em}{\mathrm{cm}}^{-1}.$ For a strongly underdoped ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.5}$ crystal with $T_c=52\mathrm{}\hspace{0.5em}\mathrm{K}$ 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 $T^{*}\sim 150\hspace{0.5em}\mathrm{K}.$ 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 ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6.75}$ crystal with $T_c=80\mathrm{}\hspace{0.5em}\mathrm{K},$ 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.