Raman study of the variations of the Jahn-Teller distortions through the metal-insulator transition in magnetoresistive La0.7Ca0.3MnO3 thin films

The polarized Raman spectra of La0.7Ca0.3MnO3 magnetoresistive thin films d eposited on LaAlO3, SrTiO3, YSZ, and MgO substrates were studied in a broad temperature range including T-c. Depending on variations of their intensit y and linewidth upon cooling, the Raman bands could be classified into two groups. The lines corresponding to Raman allowed modes of the rhombohedral (or orthorhombic) structure grow in intensity and become narrower at low te mperatures. In contrast, the broad Raman bands centered near 470, 485 and 6 10 cm(-1) strongly decrease in intensity below T-c and display a correlatio n to the resistivity of the sample. We argue that these bands correspond to lattice vibrations activated by the dynamic Jahn-Teller distortions and by presence of small polarons around the Mn3+ sites. The correlation between polaron lifetimes and phonon Linewidth is also discussed.