Origin of unconventional piezoelectricity by flat phonon bands in HfO₂

Ferroelectric materials are receiving attention in the electronics industry as candidate materials for ferroelectric random-access memories (FeRAM) or nonvolatile resistance memories. In particular, HfO₂ has been studied a lot recently because it has persistently robust ferroelectricity in thin films in contrast to other conventional ferroelectrics (e.g., perovskite oxides like BaTiO₃). However, studies on the electromechanical reaction of HfO₂ have not yet progressed significantly in the physical aspect. Recently, our group showed that HfO₂ can be employed as the possibly highest density memory material due to its unusual phonon flat band phenomenon [1]. In this talk, we will discuss the effect of the flat-bands on ferroelectric switching and unconventional piezoelectric effects. First, unlike conventional ferroelectrics which have a large coupling between ferroelectric switching and lattice constants, we discovered a surprising phenomenon that the lattice constants do not change at all during the switching. This is mainly because the ferroelectricity of  HfO₂ is strongly related to the intrinsic effect of the flat bands which makes the velocity of the phonon propagation close to zero. The weak interaction of polar phonon and mechanical lattice expansion will help to overcome deleterious problems in ferroelectric devices such as fatigue usually caused by repetitive switching. Furthermore, we will talk about the origin of the recently reported negative piezoelectricity [2] through DFT combined with a full phonon Hamiltonian and seek experimental routes to maximize the unconventional piezoelectricity by using doping or epitaxial strain.

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

[2] “Piezoelectricity in hafnia” Dutta et al., https://arxiv.org/abs/2107.07414