Ferroelectric metals in 1T/1T′-phase transition metal dichalcogenide MTe2 bilayers (M = Pt, Pd, and Ni)

Ferroelectricity and metallicity cannot coexist due to the screening effect of conducting electrons, and a large number of stable monolayers with $1\mathrm{T}/1{\mathrm{T}}^{\ensuremath{'}}$ phase lack spontaneous polarization due to inversion symmetry. In this work, we have constructed the $\ensuremath{\pi}$-bilayer structures for transition metal dichalcogenides ($M{\mathrm{Te}}_{2},M=\mathrm{Pt}$, Pd, and Ni) with van der Waals stacking, where two monolayers are related by ${C}_{2z}$ rotation, and have demonstrated that these $\ensuremath{\pi}$ bilayers are typical ferroelectric metals (FEMs). The $\ensuremath{\pi}$-bilayer structure widely exists in nature, such as $1{\mathrm{T}}^{\ensuremath{'}}/{\mathrm{T}}_{d}\text{\ensuremath{-}}\mathrm{TMD}$, $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Bi}}_{4}{\mathrm{Br}}_{4}$. The computed vertical polarization of ${\mathrm{PtTe}}_{2}$ and ${\mathrm{MoTe}}_{2}\phantom{\rule{4pt}{0ex}}\ensuremath{\pi}$ bilayers are 0.46 and 0.25 pC/m, respectively. We show that the switching of polarization can be realized through interlayer sliding, which only requires crossing a low energy barrier. The interlayer charge transfer is the source of both vertical polarization and metallicity, and these properties are closely related to the spatially extended Te-${p}_{z}$ orbital. Finally, we reveal that electron doping can significantly adjust the vertical polarization of these FEMs in both magnitude and direction. Our findings introduce a class of FEMs, which have potential applications in functional nanodevices such as ferroelectric tunneling junction and nonvolatile ferroelectric memory.