Separated Electron-Phonon and Phonon-Phonon Scatterings Across Interface in Thin Film LaCoO₃ /SrTiO₃

Electron-phonon coupling (EPC) and phonon-phonon scattering (PPS) are at the core of the microscopic physics mechanisms of vast quantum materials. However, to date, there are rarely reports that these two processes can be spatially separated, although they are usually temporally detached with different characteristic lifetimes. Here, by employing ultrafast spectroscopy to investigate the photo-carrier ultrafast dynamics in a LaCoO₃ thin film on a (100) SrTiO₃ substrate, intriguing evidence is found that the two interactions are indeed spatially separated. The EPC mainly occurs in the thin film, whereas PPS is largely in the substrate, especially at the several atomic layers near the interface. Across-interface penetration and decay of optical phonons into acoustic phonons thus naturally occur. An EPC strength λ_Eg  = 0.30 is also obtained and an acoustic phonon mode at 45.3 GHz is observed. The finding lays out a cornerstone for future quantum nano device designs.