Ferroelectricity in charge-density-wave 1T−TiX2 ( X = S, Se, and Te) bilayers

Exploration of multiphase coexistence has always been an important topic in condensed matter physics, which can produce rich physics to meet the needs of modern science and technology. In this work, by means of first-principles calculations, we successfully predicted the ferroelectricity in the charge density wave (CDW) material 1T−TiX2 (X = S, Se, and Te) bilayers by stacking two van der Waals (vdW) monolayers. Because the CDW transition occurring at low temperatures is accompanied by a metal-insulator transition (MIT), the ferroelectric (FE) polarization also changes. Taking 1T−TiSe2 as an example, its bilayer behaves as an FE metal in the high-temperature normal phase, in which ferroelectricity and metallicity can coexist due to the dispersed spatial distribution of polarized and conducting electrons, with vertical polarization up to 0.38 pC/m. After the CDW transition, these bilayers exhibit semiconductivity as a result of the periodic lattice distortion. Meanwhile, the shielding effect of electrons is removed, and the polarization is approximately tripled. Interestingly, we found four polarization states in the CDW phase and confirmed that multistate modulation can be achieved by interlayer sliding with low energy consumption. Our findings link the CDW transition to ferroelectricity, which has potential applications in functional nanodevices and provides a new route to design polarization multistate for developing next-generation memory devices. locked icon locked icon locked icon locked icon locked icon locked icon locked icon locked icon locked icon locked icon locked icon locked icon locked icon locked icon locked icon locked icon locked icon locked icon locked icon locked icon locked icon Physics Subject Headings (PhySH)Charge density wavesMetal-insulator transitionFerroelectricsVan der Waals systemsFirst-principles calculations