Raman scattering study of Nd_1−xSr_xMnO₃ (x = 0.3, 0.5)

We report on polarized Raman scattering of single crystals of Nd_1−xSr_xMnO₃ (x = 0.3, 0.5). Raman spectra of Nd_0.7Sr_0.3MnO₃ show a significant change through the metal–insulator transition. In the ferromagnetic metallic phase phonon modes grow in intensity and number while the electronic continuum becomes more pronounced. We suggest that these effects are due to the strong competition between the localization and the delocalization of carriers which is the origin of the largest colossal magnetoresistance effect ever reported for the manganites. Raman spectra of Nd_0.5Sr_0.5MnO₃, upon cooling through the charge-ordering temperature T_CO = 148 K, exhibit several new lines which undergo a substantial hardening. This hardening is interpreted as a freezing of the Jahn–Teller distortions with a gradual decrease of a fraction of the ferromagnetic phase in the CE-type charge/orbital ordered state.