First-principles Studies of Ferroelectricity in $ \rm BiMnO_3 $ Thin Films*

The ferroelectricity in $ \rm BiMnO_3 $ thin films is a long-standing problem. We employed a first-principles density functional theory with inclusion of the local Hubbard Coulomb (U) and exchange (J) terms. The parameters U and J are optimized to reproduce the atomic structure and the energy gap of bulk C2/c $ \rm BiMnO_3 $. With these optimal U and J parameters, the calculated ferromagnetic Curie temperature and lattice dynamics properties agree with experiments. We then studied the ferroelectricity in few-layer $ \rm BiMnO_3 $ thin films on $ \rm SrTiO_3 $(001) substrates. Our calculations identified ferroelectricity in monolayer, bilayer and trilayer $ \rm BiMnO_3 $ thin films. We find that the energy barrier for $90^\circ$ rotation of electric polarization is about 3 -- 4 times larger than that of conventional ferroelectric materials.