Investigation of the effect of polarization on the phonon modes in thin film ferroelectric material*

Thin-film ferroelectric materials provide promising solutions for information storage and transfer such as ferroelectric random-access memory (FeRAMs) where phonons can have a significant impact on emergent properties such as novel thermal and electronic conducting channels, atomic scale phononic filtering, localization, and chirality. However, atomic scale behavior of phonon properties at abrupt interfaces and domain walls is not well understood. We use scanning transmission electron microscopy (STEM) coupled with momentum-resolved vibrational electron energy-loss spectroscopy (vib-EELS) to understand the correlation between phonons and atomic displacement in a prototypical thin film PbTiO₃ (on DyScO₃ substrate) with the variation of c and a polarization. Our experiments indicate a shift of 6 meV in one of phonon modes at 62 meV in ferroelectric PbTiO₃ compared to the paraelectric SrTiO₃ sample. This implies polarization can, indeed, change the phonon modes. Further study is needed to fully understand how the phonons modes shift due to polarization.