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 1T/1T' phase lack spontaneous polarization due to inversion symmetry. In this work, we have constructed the pi-bilayer structures for transition metal dichalcogenides (MTe2, M = Pt, Pd, and Ni) with van der Waals stacking, where two monolayers are related by C2z rotation, and have demonstrated that these pi bilayers are typical ferroelectric metals (FEMs). The pi-bilayer structure widely exists in nature, such as 1T'/Td-TMD, alpha-Bi4Br4. The computed vertical polarization of PtTe2 and MoTe2 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-pz 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.