Oxygen isotope effect and phase separation in the optical conductivity of (La0.5Pr0.5)0.7Ca0.3MnO3 thin films

The optical conductivities of films of (La0.5Pr0.5)0.7Ca0.3MnO3 with different oxygen isotopes (O16 and O18) have been determined in the spectral range from 0.3 to 4.3 eV using a combination of transmission in the midinfrared and ellipsometry from the near-infrared to ultraviolet regions. We have found that the isotope exchange strongly affects the optical response in the ferromagnetic phase in a broad frequency range, in contrast to the almost isotope-independent optical conductivity above TC. The substitution by O18 strongly suppresses the Drude response and a midinfrared peak while enhancing the conductivity peak at 1.5 eV. A qualitative explanation can be given in terms of the phase separation present in these materials. Moreover, the optical response is similar to the one extracted from measurements in polished samples and other thin films, which signals to the importance of internal strain.