Raman study of the copper isotope effect in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$

Raman spectra of isotopically pure ${\mathrm{YBa}}_2^{63}$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ and ${\mathrm{YBa}}_2^{65}$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ were taken at room temperature and at 20 K. We find that at room temperature the phonon line at 151 ${\mathrm{cm}}^{\mathrm{-}1}$ shows a shift in frequency of 2.0±0.3 ${\mathrm{cm}}^{\mathrm{-}1}$ with Cu isotopic substitution, while the phonon at 116 ${\mathrm{cm}}^{\mathrm{-}1}$ does not shift within the experimental accuracy (0.05±0.3 ${\mathrm{cm}}^{\mathrm{-}1}$). Similar values are also obtained at 20 K. This proves that the phonon line at 151 ${\mathrm{cm}}^{\mathrm{-}1}$ has predominantly Cu character and that the coupling between the Ba and Cu sublattice vibrations is very weak. Fits to the experimental data with a coupled two-mode model yield a value for the Ba admixture of the 151 ${\mathrm{cm}}^{\mathrm{-}1}$ phonon of less than 20% and only a weak renormalization of the bare Ba and Cu phonon frequencies.