Two-dimensional ferroelectric crystal with temperature-invariant ultralow thermal conductivity

We report the discovery of temperature-invariant ultralow thermal conductivity ($\ensuremath{\kappa}$) in monolayer ${\ensuremath{\beta}}^{\ensuremath{'}}\text{\ensuremath{-}}{\mathrm{In}}_{2}{\mathrm{Se}}_{3}$, a two-dimensional ferroelectric crystal with in-plane polarization. Using a combination of generalized Wigner transport equation theory and machine-learning-assisted molecular dynamics simulations, we reveal that the balance between particlelike phonon propagating and wavelike tunneling transport mechanisms results in a propagating-tunneling-invariant ultralow thermal conductivity of approximately 0.6 W/mK (comparable to that of glass) over a broad temperature range ($150<T<800$ K). This behavior stems from intrinsic strong lattice anharmonicity driven by ferroelectric dipolar fluctuations, eliminating the need for extrinsic structural modifications. In contrast, the $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{In}}_{2}{\mathrm{Se}}_{3}$ monolayer, which shares the same stoichiometry, exhibits a temperature-dependent thermal conductivity typical of simple crystals. We show that the anharmonicity in ${\ensuremath{\beta}}^{\ensuremath{'}}\text{\ensuremath{-}}{\mathrm{In}}_{2}{\mathrm{Se}}_{3}$ can be precisely modulated by an external electric field, enabling on-demand control of the temperature scaling behavior of heat conductivity. Furthermore, an electric-field-driven motion of the $\ensuremath{\alpha}/{\ensuremath{\beta}}^{\ensuremath{'}}$ phase interface is demonstrated, supporting a nonvolatile, large thermal switching ratio of $>3$. These findings provide fundamental insights into the interplay between field-tunable lattice anharmonicity, phonon dynamics, and thermal transport mechanisms.