Ferroelectricity with Asymmetric Hysteresis in Metallic LiOsO_{3} Ultrathin Films

Bulk LiOsO_{3} was experimentally identified as a "ferroelectric" metal where polar distortions coexist with metallicity [Shi et al., Nat. Mater. 12, 1024 (2013)NMAACR1476-112210.1038/nmat3754]. 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 LiOsO_{3} 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 the external electric field whereas the field-induced force on inner Li atoms is nearly fully screened by itinerant electrons. As a relevant by-product of our study, we also extend the concept of "Born effective charge" to finite metallic systems, and show its usefulness to rationalize the observed effects.