Far-infrared c-axis conductivity of flux-grown ${\mathrm{Y}}_{1-x}{\mathrm{Pr}}_x{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_7$ single crystals studied by spectral ellipsometry

The far-infrared c-axis conductivity of flux-grown ${\mathrm{Y}}_{1-x}{\mathrm{Pr}}_x{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_7$ single crystals with $0.2<~x<~0.5$ has been studied by spectral ellipsometry. We find that the c-axis response exhibits spectral features similar to deoxygenated underdoped ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta },$ i.e., a pseudogap develops in the normal state, the phonon mode at $320{\mathrm{cm}}^{-1}$ exhibits an anomalous T dependence, and an additional absorption peak forms at low temperature. This suggests that the $T_c$ suppression in flux grown Pr-substituted crystals is caused by a decrease of the hole content and/or by carrier localization rather than by pair breaking.