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

The far-infrared c-axis conductivity of flux-grown Y${}_{1-x}$Pr${}_x$Ba${}_2$Cu${}_3$O${}_7$ single crystals with 0.2$\le$x$\le$0.5 has been studied by sepectral ellipsometry. We find that the c-axis response exhibits spectral features similar to deoxygenated underdoped YBa${}_2$Cu${}_3$O${}_{7-\delta }$, i.e., a pseudogap develops in the normal state, the phonon mode at 320 cm${}^{-1}$ exhibits an anomalous T-dependence and an additional absorption peak forms at T. 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.