  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.
