Anomalous ferroelectricity and double-negative effects in bilayer hexagonal boron nitride

Recently, the presence of an out-of-plane ferroelectric polarization was experimentally confirmed in bilayer two-dimensional (2D) hexagonal boron nitride (h-BN). In this work, we employ first-principles computations to reveal that (i) such slidetronic ferroelectricity is electronic in nature and arises from in-plane interlayer translation, which is distinctive among 2D slidetronic ferroelectric materials; and (ii) there is a coexistence of a negative longitudinal piezoelectric effect and an out-of-plane negative Poisson's ratio in the ferroelectric state of bilayer h-BN, which we refer to as anomalous double-negative effects.