Renormalization of phonons in a (Y/Pr)${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ superlattice investigated by Raman spectroscopy

Raman measurements on a superconducting superlattice with a 2:1 sequence of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ and ${\mathrm{PrBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ layes revealed a new line in the spectral region around 320 ${\mathrm{cm}}^{\mathrm{-}1}$, which is interpreted as a combination of odd lattice vibrations on the Y sublayers with a partial admixture of an even parity mode on the Pr sublayer, becoming Raman active in the superstructure. Its eigenvector is similar to a phonon from the interior of the Brillouin zone of the original lattice. No superconductivity-induced self-energy effects are observed for this phonon, although the IR-active counterparts of the individual Y sublayer vibrations in ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ exhibit those effects.