Optical phonons in superconducting Sm _1.85Ce _0.15CuO ₄ single crystals

We report Raman and infrared measurements of optical phonons in superconducting Sm _1.85Ce _0.15CuO ₄ single crystals (T _c = 15 K). All zone-center phonon modes predicted by a factor-group analysis have been identified, except for the low-frequency E _g mode involving Sm(Ce) atoms. Two additional c-axis modes with A _1g symmetry were found at 134 cm ^-1 and 591 cm ^-1 in the Ramari spectra. We identify the 134 cm ^-1 mode as the lowest-frequency infrared-active c-axis A _2u mode, mixed into the Raman spectra due to disorder via a breakdown of the usual exclusive selection rule. The 591 cm ^-1 Raman mode is found to be closely connected with an extra infrared mode observed at the same frequency. Our measurements on Sm _2-xCe _xCuO ₄ (x = 0.0 - 0.18) ceramics show, for the first time, that the strong Raman peak at 591 cm ^-1 appears only in the narrow range of Ce-doping, 0.1 ≤ x ≤ 0.17, which nearly coincides with the Ce-doping range where the Sm _2-xCe _xCuO ₄ system is superconducting at low temperatures.