Title: Ferroelectric metals in van der Waals bilayers

Abstract: The combination of metallicity and ferroelectricity breaks traditional boundaries, paving new avenues for innovative electronic materials and devices. This breakthrough is particularly notable, as metallicity and ferroelectricity have traditionally been considered mutually exclusive physical properties. In this work, starting with non-polar metallic single layers, we propose a general scheme for designing 2D ferroelectric metals (FEMs) based on van der Waals interaction. By first-principles calculations, we further substantiate the feasibility of the design scheme in real materials such as FeSe and H-MnTe2. Notably, this scheme unveils unique metallic ferroelectricity, characterized by reversing polarization through interlayer sliding. Furthermore, the combination of inherent magnetism with sliding ferroelectricity leads to multiferroicity. The investigated design scheme and observed phenomena have broad applicability across 2D materials. Our results not only pave the way for research in 2D FEMs but also offer promising prospects for foundational studies of coupled physical phenomena in 2D lattices.