Ferroelectric metals in 1T/1T'-phase transition metal dichalcogenide MTe2 bilayers (M = Pt, Pd, and Ni)

Abstract

Ferroelectricity and metallicity cannot coexist due to the screening effect of conducting electrons, and a large number of stable monolayers with 1T/1T${}^{\prime }$ phase lack spontaneous polarization due to inversion symmetry. In this work, we have constructed the $\pi$-bilayer structures for transition metal dichalcogenides ($M$Te${}_2,M=$ Pt, Pd, and Ni) with van der Waals stacking, where two monolayers are related by $C_{2z}$ rotation, and have demonstrated that these $\pi$ bilayers are typical ferroelectric metals (FEMs). The $\pi$-bilayer structure widely exists in nature, such as 1T${}^{\prime }$/T${}_d$-TMD, $\alpha$-Bi${}_4$Br${}_4$. The computed vertical polarization of PtTe${}_2$ and MoTe${}_2$ $\pi$ 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.

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