Fractional-Quantum Ferroelectrics: A Route to High-Mobility Ferroelectric Semiconductors

Abstract: Ferroelectric semiconductors are promising for multifunctional electronics, yet their typically low carrier mobilities remain a major limitation. Using first-principles phonon-limited transport calculations for monolayer In$_2$Se$_3$, we show that this limitation depends critically on the microscopic origin of ferroelectricity. In displacive $β&#39;$-In$_2$Se$_3$, low-frequency ferroelectric modes dominate carrier scattering, with additional contributions from longitudinal-optical (LO) phonons, limiting the room-temperature electron mobility to a few cm$<sup>2$/(V s). By contrast, in fractional-quantum ferroelectric $α$-In$_2$Se$_3$, ferroelectric-mode scattering is absent because polarization arises from discrete lattice-scale atomic displacements rather than soft-mode condensation. Transport is therefore dominated by LO phonons, yielding a room-temperature mobility above 70 cm$<sup>2$/(V s). Carrier doping further screens long-range electron-LO-phonon interactions and raises the mobility beyond 300 cm$<sup>2$/(V s) at experimentally accessible densities. These results establish that fractional-quantum ferroelectricity can decouple robust polarization from strong intrinsic carrier scattering, offering a route toward high-mobility ferroelectric semiconductors.