Orbital distortion and electric field control of sliding ferroelectricity in a boron nitride bilayer.

Recent experiments confirm that two-dimensional (2D) boron nitride (BN) films possess room-temperature out-of-plane ferroelectricity when each BN layer is sliding with respect to each other. This ferroelectricity is attributed to the interlayered orbital hybridization or interlayer charge transfer in previous work. In this work, we attempt to understand the sliding ferroelectricity from the perspective of orbital distortion of long-pair electrons. Using the maximally localized Wannier function (MLWF) method and first-principles calculations, the out-of-plane pz orbitals of BN are investigated. Our results indicate that the interlayer van der Waals interaction causes the distortion of the N pz orbitals. Based on the picture of out-of-plane orbital distortion, we propose a possible mechanism to tune the ferroelectric polarization by external fields, including electric field and stress field. It is found that both the polarization intensity and direction can be modulated under the electric field. The polarization intensity of the system can also be controlled by stress field perpendicular to the plane. This study will provide theoretical help in the device design based on sliding ferroelectrics.&#xD.