Raman study of (YBa2Cu3O7)m(PrBa2Cu3O7)nsuperlattices

We report Raman-scattering measurements on several superconducting superlattice films of (${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ ${)}_{\mathit{m}}$(${\mathrm{PrBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ ${)}_{\mathit{n}}$. The superstructure period varied between 4<(m+n)<12 and was oriented with the c axis of the initial bulk crystalline materials lying normal to the film surface. The observed Raman-active vibrations of the superlattice are readily identified through comparison with spectra from the bulk components. The dependence of the vibrational frequencies on m and n shows that phonons associated with the apical oxygen atoms (${\mathrm{O}}_4$) are mainly localized to the Y and Pr layers for the larger period structures and transform to an extended excitation as the modulation period is reduced. This behavior is consistent with charge redistribution across the interface and may account for the drop in the transition temperature observed in thin Y-layer superlattices. The degree of interdiffusion between Pr and Y sites is shown to be negligible with no detectable interfacial alloying. Reminiscent of behavior in bulk ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$, the out-of-phase O(2)-O(3) phonon confined to Y layers as small as two unit cells softens below the superconducting transition temperature ${\mathit{T}}_{\mathit{c}}$. The corresponding phonon localized to the Pr layers shows no softening down to n=1 and therefore suggests the absence of proximity-induced superconductivity in the ${\mathrm{PrBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ layers.