CHARGE-TRANSFER IN HIGH-T-C (Y PR)BA2CU3O7 SUPERLATTICES/

We comment on the charge-transfer model for the unique softening of th e apex [O(4)] oxygen observed in Raman spectra of a- and c-oriented (Y Ba2Cu3O7)(m)(PrBa2Cu3O7)(n) superlattices presented in a recent paper by Ham et al. [Phys. Rev. B 50, 16 598 (1994)]. We find the size of th e charge transfer necessary for the observed shift too large to explai n the observations. In addition, a conclusion about the unique behavio r of both types of superlattices can only be drawn if the sample quali ty is properly considered.