Ferroelectricity and metallicity cannot coexist due to the screening effect of conducting electrons. Ferroelectric metals (FEMs) are rare in nature but have demonstrated various fascinating properties, such as unconventional superconductivity, unique optical responses, and magnetoelectric effects. The transition metal dichalcogenides (TMD) have a stable layered 1T/1T' phase and lack spontaneous polarization due to inversion symmetry. Due to unique interlayer interaction, the mechanism of sliding ferroelectricity in van der Waals (vdW) layered materials was proposed by researchers in 2017. Recently, researchers have directly observed ferroelectric switching in multilayer WTe₂ semimetal, drawing much attention to interlayer sliding ferroelectricity. The multilayer structure of TMD materials provides an ideal platform for the study of sliding ferroelectricity and metallic ferroelectricity.

Recently, the group led by Prof. Zhijun Wang from the Institute of Physics of the Chinese Academy of Sciences, constructed the π bilayer structure for MTe₂ (M = Pt, Pd, and Ni) family TMD, with vdW stacking, where two monolayers are related by C_2z rotation, and reported that these π bilayers are typical FEMs [1], as shown in Fig. 1. Based on first-principles calculations, several results have been demonstrated. First, although the 1T-MTe₂ monolayer is insulating, the bilayer becomes metallic. Second, the two ferroelectric phases of the π bilayers are with relative displacement v=(1/3,2/3) and (2/3,1/3), respectively, and the ferroelectric polarization is out of plane. Third, the switching of vertical polarization can be realized through interlayer sliding, which only requires crossing a low energy barrier. Finally, electron doping can significantly adjust the vertical polarization of these π-bilayer FEMs in both magnitude and direction. In addition, we show that 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. The π-bilayer structure widely exists in nature, such as 1T'/T_d-MoTe₂ (superconductivity induced by carrier doping), α-Bi₄Br₄ (QSHI in monolayer), which enables us to study the interplay between the ferroelectricity and other novel properties (such as superconductivity and topology).

Fig. 1 MTe₂ π bilayer (a)-(b) structure, (c)-(e) energy and polarization surface, (d)-(f) energy barrier and polarization on ferroelectric switching pathway, (g) Schematic diagram of the π bilayer structure., (h) relationship between polarization and doping.

The results predict a class of metallic ferroelectrics with potential applications in functional nanodevices. This work has been published in Physical Review B with the title "Ferroelectric metals in 1T/1T'-phase transition metal dichalcogenide MTe₂ bilayers (M = Pt, Pd, and Ni)". It was supported by projects from the National Natural Science Foundation of China, the Chinese Academy of Sciences, and the Center for Materials Genome.

Fig. 2 (a) Crystal structure of the PtTe_1.75 monolayer, (b) projected band structure without spin-orbit coupling, (c) band structure with spin-orbit coupling and Z₂ topological number corresponding to the bandgap, and (d) Schematic diagram of hydrogen evolution reaction process on PtTe_1.75 monolayer.

In addition, this research group also reports that the PtTe₂ material with well-ordered Te vacancy (PtTe_1.75) is an unconventional two-dimensional material with large and tunable spin Hall conductivity and excellent hydrogen evolution reaction activity [2], as shown in Figure 2. The result offers a new idea to search two-dimensional materials with large spin Hall conductivity, i.e., by introducing inversion–symmetry breaking vacancies in large spin-orbit coupling systems. The relevant results have been published in Research with the title "Large Spin Hall Conductivity and Excellent Hydrogen Evolution Reaction Activity in Unconventional PtTe_1.75 Monolayer" (Research, 2023,6:0042).