Giant quartic-phonon decay in PVD-grown α-MoO3 flakes

Elementary excitations, such as in-plane anisotropic phonons and phonon polaritons (PhPs), in α-MoO3 play key roles in its outstanding physical properties like high carrier mobility and ultralow phonon thermal conductivity (κp). Understanding the excitation mechanisms like phonon-phonon interactions is the most fundamental step to further applications. Here, we report on the systematic Raman investigations on phonon anisotropy and anharmonicity of representative Mo-O stretching vibration phonon modes (SVPMs) in physical vapor deposition (PVD)-grown α-MoO3 flakes. Polarizations of SVPMs verify the phonon anisotropy. The abnormal temperature dependence of SVPMs reveals that giant quartic-phonon decay dominates the phonon anharmonicity in α-MoO3. An ultrashort phonon lifetime of ∼ 0.34 ps gives evidence of theoretically predicted ultralow κp in α-MoO3. Our findings give deep insight into the phonon-phonon interactions in a-MoO3 and provide an indicator for its extreme thermal device applications.