Scale-free ferroelectricity induced by flat phonon bands in HfO2

The atomic-scale structure of hafnium oxide explains ferroelectric properties for very thin films. Switching to the atomic scale Ferroelectric materials are attractive because they provide a way to change electrical resistance by using an electric field. Lee et al. used simulations to explain the persistence of ferroelectric behavior in very thin films of hafnium oxide (see the Perspective by Noheda and Íñiguez). The authors' calculations show that ferroelectric properties should be found in films below 1 nanometer thick. This makes the material very attractive for the next generation of random access memory. Science, this issue p. 1343; see also p. 1300 Discovery of robust yet reversibly switchable electric dipoles at reduced dimensions is critical to the advancement of nanoelectronics devices. Energy bands flat in momentum space generate robust localized states that are activated independently of each other. We determined that flat bands exist and induce robust yet independently switchable dipoles that exhibit a distinct ferroelectricity in hafnium dioxide (HfO2). Flat polar phonon bands in HfO2 cause extreme localization of electric dipoles within its irreducible half-unit cell widths (~3 angstroms). Contrary to conventional ferroelectrics with spread dipoles, those intrinsically localized dipoles are stable against extrinsic effects such as domain walls, surface exposure, and even miniaturization down to the angstrom scale. Moreover, the subnanometer-scale dipoles are individually switchable without creating any domain-wall energy cost. This offers unexpected opportunities for ultimately dense unit cell–by–unit cell ferroelectric switching devices that are directly integrable into silicon technology.