Optical phonons in superconducting Sm1.85Ce0.15CuO4 single crystals

We report Raman and infrared measurements of optical phonons in superconduc ting Sm1.85Ce0.15CuO4 single crystals (T-c = 15 K). All cone-center phonon modes predicted by a factor-group analysis have been identified, except for the low-frequency Eg mode involving Sm(Ce) atoms. Two additional c-axis mo des with A(1g) symmetry were found at 131 cm(-1) and 591 cm(-1) in the Ramm an spectra. We identify the 134 cm(-1) mode as the lowest-frequency infrare d-active c-axis Az, mode, mixed into the Raman spectra due to disorder via a breakdown of the usual exclusive selection rule. The 591 cm(-1) Raman mod e is found to be closely connected with an extra infrared made observed at the same frequency. Our measurements on Sm2-xCexCuO4 (x = 0.0 - 0.18) ceram ics show, for the first time, that the strong Raman peak at 591 cm(-1) appe ars only in the narrow range of Ce-doping, 0.1 less than or equal to x less than or equal to 0.17, which nearly coincides with the Ce-doping range whe re the Sm2-xCexCuO4 system is superconducting at low temperatures.