Origin of Zero-Energy-Cost Domain Walls beyond the Flat Phonon Bands

Although we report our purely DFT paper on the zero-energy-cost domain walls in HfO2 [1], still the flat polar phonon band is not the entire origin of the zero-energy-cost domain wall. This is because other phonons modes except the polar mode included in this compound are not energetically flat at all. Finally, we discover that a new-type phonon-phonon coupling can successfully explain the zero-energy-cost domain walls in this fluorite-type ferroelectric. I will talk about the origin by introducing the new coupling mechanism that uniquely exists in this commercial ferroelectric.

 [1] Scale-free ferroelectricity induced by flat phonon bands in HfO2, H.-J.Lee et al., Science 369, 1343 (2020).