RAMAN-STUDY OF THE COPPER ISOTOPE EFFECT IN YBA2CU3O7-DELTA

Raman spectra of isotopically pure YBa263Cu3O7-δ and YBa265Cu3O7-δ were taken at room temperature and at 20 K. We find that at room temperature the phonon line at 151 cm-1 shows a shift in frequency of 2.0±0.3 cm-1 with Cu isotopic substitution, while the phonon at 116 cm-1 does not shift within the experimental accuracy (0.05±0.3 cm-1). Similar values are also obtained at 20 K. This proves that the phonon line at 151 cm-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 cm-1 phonon of less than 20% and only a weak renormalization of the bare Ba and Cu phonon frequencies. © 1995 The American Physical Society.