Flat-Band Ferroelectricity for Densest Memory Fist Discovered in HfO2

Since ferroelectricity is a collective phenomenon requiring the simultaneous displacement o thousands of atoms, it’s been believed since it’s discovery in 1920 that finite-sized domains (10~100nm) are required to stabilize and switch the ferroelectric dipole moments. Here, we break the 100-years belief if we introduce flat-band physics into the history of ferroelectricity. As flat energy band in momentum space has been already known to induce the localized states in real space and to cause very unusual phenomena such as graphene superconductivity and electron lattices, we discovered that, for the first time, flat bands in polar phonon exist surprisingly in the commercial ferroelectric HfO₂ and showed that they induce irreducibly localized dipoles of a few angstroms in real space. More strikingly, these extremely localized dipoles are individually stable and switchable by external electric field and thus now one circumvents the formation of the conventional large domains for the ferroelectric dipole switching. We can directly switch the ultimately-small unit-cell-scale dipoles and finally pave a way to achieve densest memory devices by employing the fundamental flat-band physics in the most commercial ferroelectric compatible with Si-technology [1].<br/>[1] “<i>Scale-free ferroelectricity induced by flat phonon bands in HfO₂</i>” Lee <i>et al</i>., Science 369, 1343 (2020).