Superlattices using Atomic-resolution Microscopy and Theory*

Perovskite oxides offer an abundance of properties with technological importance that are tunable by creating superlattices (SLs). Most research so far focuses on electronic properties, but such SLs can also feature unique thermal properties, which are dictated by lattice vibrations [1]. Here we combine atomic-resolution scanning-transmission-electron microscopy, vibrational electron energy-loss spectroscopy, and density-functional-theory calculations to probe the phonon spectra of a series of SrTiO₃-CaTiO₃ SLs with 1 to 27 unit-cells per layer. We show large-period SLs feature bulk plus localized interface vibrational modes. As layer thickness decreases, the bulk modes disappear while the interface modes underpin an emergent phonon structure, which is also reflected in the macroscopic vibrational response. This behavior correlates to period thickness reaching the length scale of octahedral coupling at the interfaces, thus reflecting the dominant role of interfaces as size decreases and the need for spatially quantified structural and vibrational data to enable complete understanding of SL behavior.
[1] J. Ravichandran et al., Nature Materials (2014).