Giant Manipulation of Phonon Hydrodynamics in Ferroelectric Bilayer Boron Nitride at Room Temperature and Beyond

Phonon hydrodynamics is an intriguing thermal transport mechanism that offers a great opportunity for phonon manipulation and thermal management. For decades, it has been believed that phonon hydrodynamics occurs only at very low temperatures, until very recently, it was confirmed that phonons can efficiently flow in low-dimensional materials at room temperature. However, to date, no study has been reported on the manipulation of phonon hydrodynamics at room temperature via reversible methods such as an external electric field. Motivated by the spontaneous out-of-plane polarization of ferroelectric bilayer boron nitride (BN), we investigate the effect of in-plane electric fields on phonon transport. With the electric field slightly switched on, the lattice thermal conductivity of bilayer BN steeply increases with a maximum augmentation factor of & SIM;2, and the peak thermal conductivity reaches 840 W/mK. Such a colossal change stems from the dominant phonon hydrodynamics. By tuning the external electric field, the phonon hydrodynamics can be manipulated to contribute 50% of the overall thermal transport in bilayer BN even at room temperature, indicating robust manipulation of phonon hydrodynamics. Over a broad frequency range (< 10 THz), the Normal (N) phonon transport is 2 orders of magnitude stronger than that of the resistive Umklapp process. By analyzing mode level phonon behavior, we reveal that such enhancement is mainly contributed by the two low-frequency phonon branches (out-of-plane ZA and low-lying ZO1). Under the in-plane electric field, both the number of charges and charge distribution around the nitrogen atom can be largely altered, which is the natural response of the spontaneous electric polarization as a ferroelectric material, leading to phonon renormalization and modulation of phonon anharmonicity. Our study paves the way for dynamically controlling phonon hydrodynamics in layered ferroelectrics at room temperature and beyond without altering the atomic structure and would have a significant impact on emerging applications.

Automated summary

Phonon hydrodynamics can efficiently flow in low-dimensional materials at room temperature. Our study reveals the manipulation of phonon hydrodynamics in layered ferroelectric materials using an external electric field, which can contribute up to 50% of the overall thermal transport.