Raman light scattering on ultra-thin films of LaNiO₃ under compressive strain

In order to investigate the lattice dynamics of very thin films, we have carried out a series of depth-resolved Raman scattering measurements. Combining the narrow depth-of-field of confocal Raman spectroscopy with vertical scans of the sample, we have obtained high quality phonon spectra of fully strained LaNiO₃ films with thickness as low as 4 pseudocubic unit cells. With increasing film thickness a large softening of a LaNiO₃E_g-bending mode is observed, indicating the relaxation of biaxial compressive strain imposed by the LaAlO₃ substrate