Ferroelectricity with Asymmetric Hysteresis in Metallic LiOsO3 Ultrathin Films

Abstract

Bulk LiOsO3 was experimentally identified as a "ferroelectric" metal where polar distortions coexist with metallicity [Shi et al., Nature Materials 12, 1024]. It is generally believed that polar displacements in a "ferroelectric" metal cannot be switched by an external electric field. Here, via comprehensive density functional theory calculations, we demonstrate that a two-unit-cell-thick LiOsO3 thin film exhibits a ferroelectric ground state having an out-of-plane electric dipole moment that can be switched by an external electric field. Moreover, its dipole moment-versus-electric-field hysteresis loop is asymmetric because only surface Li ions' displacements are reversed by external electric field whereas the field-induced force on inner Li atoms are nearly fully screened by itinerant electrons. As a relevant by-product of our study, we also extend the concept of "Born effective charge" to

Review on Recent Developments in 2D Ferroelectrics: Theories and Applications

“…For example, by the first-principles calculations, Lu et al found that the displacements of surface Li ions contributed mostly to the polarization in 2-UC-thick polarmetal LiOsO 3 thin films. [43] This happens because the Born effective charge of the surface Li ions is higher than that of the inner Li ions by more than ten times owing to the reduced compensation of the electric dipole by free charge at the surfaces ( Figure 2d).…”

“…Reproduced with permission. [43] Copyright 2019, American Physical Society. the ferroelectric polarization.…”

“…[70] In addition, Lu et al investigated the polarization properties in LiOsO 3 thin films with two to several UC thickness. [43] They found that only those films with thickness of even unit cells have a polar ground state, and only the polarization in 2-UC-thick thin films can be reverted by an external electric field. Based on the calculated Born effective charges, the formation and switching of the out-of-plane polarization can be attributed to the surface Li + ions because the electric dipoles induced by the displacement of internal Li + ions are almost completely compensated by the free charges.…”

“…It is believed that the metal and ferroelectric states are mutually exclusive for bulk materials since the external electric field can be easily screened by the intrinsic free charge. Several 2D materials have been predicted to sustain a ferroelectric metal state, such as a CrN monolayer, [ 70 ] 2‐UC‐thick LiOsO 3 thin films, [ 43 ] and a 1T′‐WTe 2 multilayer. [ 14a,71 ] Considering the suppressed movement of free charges in the vertical direction, switchable out‐of‐plane polarization can be induced in 2D materials to avoid screening from the free charge while retaining the in‐plane metallicity.…”

Prediction of Strong Converse Magnetoelectric Effect in Nb‐Doped BaTiO3‐Based Polar Metals

“…Recently, another pair of contraindicated propertiesFE and metallicityhas strongly attracted attention from the research community, which is called polar metal (or ferroelectric metal). [ 18 ] Moreover, theoretical predictions and experimental observations have confirmed that the polarization reversal can be achieved in polar metals LiOsO 3 ultrathin film, [ 19 ] Bi 5 Ti 5 O 17 , [ 20 ] and 2D WTe 2 [ 21 ] by applying an electric field. In 2015, Puggioni et al [ 22 ] predicted that (LiOsO 3 ) 1 /(LiNbO 3 ) 1 superlattice polar metal exhibits strong ME coupling as well as high ferroelectric Curie temperature (927 K) and Néel temperature (379 K).…”