Non-linear temperature dependent phononic response of epitaxial lanthanum nickelate thin film

Abstract From the temperature dependent Raman spectroscopy study (80 K–300 K) on epitaxial thin film of lanthanum nickelate (LNO), we find that the characteristics Raman active phonon modes (A1g and Eg) of LNO soften with an increase in the temperature. The extent of softening is greater for the A1g mode owing to substrate induced strain. In addition, temperature dependent Raman shifts presented with some non-linearities. Our analysis suggests that these non linearities arise due to an anharmonic contribution to the Raman shifts. We find that for A1g mode, four-phonon anharmonic process contributed most to the temperature dependent Raman shifts, while the contributions from three-phonon, strain and thermal expansion effect found to be relatively small. However, for Eg mode, the shift originated mainly from the thermal expansion effect.