Raman study of anharmonic effects in ${\mathrm{Pr}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{MnO}}_3$ thin films

We report low-temperature and hydrostatic pressure Raman measurements in ${\mathrm{Pr}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{MnO}}_3$ thin films grown on ${\mathrm{SrTiO}}_3$ and ${\mathrm{LaAlO}}_3$ substrates. From the low-temperature dependence of the modes related with the Jahn-Teller distortions of the octahedra, it is found that an increase appears in their relative intensities at temperatures close to the charge-ordering phase transition. Two rotational modes of vibration of the octahedra show an anharmonic behavior. Strains from the substrate do not induce any shift or new modes in the Raman spectra probably due to their relaxation from the interface towards the surface of the films. From the different orientation of the two films, the symmetry of certain modes has been identified.